A native New Englander, Amos Bronson Alcott was born in Wolcott, Connecticut (only recently renamed from "Farmingbury") on November 29, 1799. His parents were Joseph Chatfield Alcott and Anna Bronson Alcott. The family home was in an area known as Spindle Hill, and his father, Joseph Alcox, traced his ancestry to colonial-era settlers in eastern Massachusetts. The family originally spelled their name "Alcock", later changed to "Alcocke" then "Alcox". Amos Bronson, the oldest of eight children, later changed the spelling to "Alcott" and dropped his first name. At age six, young Bronson began his formal education in a one-room schoolhouse in the center of town but learned how to read at home with the help of his mother. The school taught only reading, writing, and spelling and he left this school at the age of 10. At age 13, his uncle, Reverend Tillotson Bronson, invited Alcott into his home in Cheshire, Connecticut, to be educated and prepared for college. Bronson gave it up after only a month and was self-educated from then on. He was not particularly social and his only close friend was his neighbor and second cousin william Alcott, with whom he shared books and ideas. Bronson Alcott later reflected on his childhood at Spindle Hill: "It kept me pure... I dwelt amidst the hills... God spoke to me while I walked the fields." Starting at age 15, he took a job working for clockmaker Seth Thomas in the nearby town of Plymouth.

At age 17, Alcott passed the exam for a teaching certificate but had trouble finding work as a Teacher. Instead, he left home and became a traveling salesman in the American South, peddling books and merchandise. He hoped the job would earn him enough money to support his parents, "to make their cares, and burdens less... and get them free from debt", though he soon spent most of his earnings on a new suit. At first, he thought it an acceptable occupation but soon worried about his spiritual well-being. In March 1823, Alcott wrote to his brother: "Peddling is a hard place to serve God, but a capital one to serve Mammon." Near the end of his life, he fictionalized this experience in his book, New Connecticut, originally circulated only among friends before its publication in 1881.

By the summer of 1823, Alcott returned to Connecticut in debt to his father, who bailed him out after his last two unsuccessful sales trips. He took a job as a schoolteacher in Cheshire with the help of his Uncle Tillotson. He quickly set about reforming the school. He added backs to the benches on which students sat, improved lighting and heating, de-emphasized rote learning, and provided individual slates to each student—paid for by himself. Alcott had been influenced by educational philosophy of the Swiss pedagogue Johann Heinrich Pestalozzi and even renamed his school "The Cheshire Pestalozzi School". His style attracted the attention of Samuel Joseph May, who introduced Alcott to his sister Abby May. She called him, "an intelligent, philosophic, modest man" and found his views on education "very attractive". Locals in Cheshire were less supportive and became suspicious of his methods. Many students left and were enrolled in the local Common school or a recently re-opened private school for boys. On November 6, 1827, Alcott started teaching in Bristol, Connecticut, still using the same methods he used in Cheshire, but opposition from the community surfaced quickly; he was unemployed by March 1828. He moved to Boston on April 24, 1828, and was immediately impressed, referring to the city as a place "where the light of the sun of righteousness has risen". He opened the Salem Street Infant School two months later on June 23. Abby May applied as his teaching assistant; instead, the couple were engaged, without consent of the family. They were married at King's Chapel on May 22, 1830; he was 30 years old and she was 29. Her brother conducted the ceremony and a modest reception followed at her father's house. After their marriage the Alcotts moved to 12 Franklin Street in Boston, a boarding house run by a Mrs. Newall. Around this time, Alcott also first expressed his public disdain for slavery. In November 1830, he and william Lloyd Garrison founded what he later called a "preliminary Anti-Slavery Society", though he differed from Garrison as a nonresistant. Alcott became a member of the Boston Vigilance Committee.

Alcott married Abby May in 1830 and they eventually had four surviving children, all daughters. Their second was Louisa May, who fictionalized her experience with the family in her novel Little Women in 1868.

Attendance at Alcott's school was falling. A wealthy Quaker named Reuben Haines proposed he and educator william Russell start a new school in Pennsylvania associated with the Germantown Academy. Alcott accepted and he and his newly pregnant wife set forth on December 14. The school was established in Germantown and the Alcotts were offered a rent-free home by Haines. Alcott and Russell were initially concerned that the area would not be conducive to their progressive approach to education and considered establishing the school in nearby Philadelphia instead. Unsuccessful, they went back to Germantown, though the rent-free home was no longer available and the Alcotts instead had to rent rooms in a boarding-house. It was there that their first child, a daughter they named Anna Bronson Alcott, was born on March 16, 1831, after 36 hours of labor. By the fall of that year, their benefactor Haines died suddenly and the Alcotts again suffered financial difficulty. "We hardly earn the bread", wrote Abby May to her brother, "[and] the butter we have to think about."

On September 22, 1834, Alcott opened a school of about 30 students, mostly from wealthy families. It was named the Temple School because classes were held at the Masonic Temple on Tremont Street in Boston. His assistant was Elizabeth Palmer Peabody, later replaced by Margaret Fuller. Mary Peabody Mann served as a French instructor for a time. The school was briefly famous, and then infamous, because of his original methods. Before 1830, writing (except in higher education) equated to rote drills in the rules of grammar, spelling, vocabulary, penmanship and transcription of adult texts. However, in that decade, progressive reformers such as Alcott, influenced by Pestalozzi as well as Friedrich Fröbel and Johann Friedrich Herbart, began to advocate writing about subjects from students' personal experiences. Reformers debated against beginning instruction with rules and were in favor of helping students learn to write by expressing the personal meaning of events within their own lives. Alcott's plan was to develop self-instruction on the basis of self-analysis, with an emphasis on conversation and questioning rather than lecturing and drill, which were prevalent in the U.S. classrooms of the time. Alongside writing and reading, he gave lessons in "spiritual culture", which included interpretation of the Gospels, and advocated object teaching in writing instruction. He even went so far as to decorate his schoolroom with visual elements he thought would inspire learning: paintings, books, comfortable furniture, and busts or portraits of Plato, Socrates, Jesus, and william Ellery Channing.

In July 1835, Peabody published her account as an assistant to the Temple School as Record of a School: Exemplifying the General Principles of Spiritual Culture. While working on a second book, Alcott and Peabody had a falling out and Conversations with Children on the Gospels was prepared with help from Peabody's sister Sophia, published at the end of December 1836. Alcott's methods were not well received; many found his conversations on the Gospels close to blasphemous. For Example, he asked students to question if Biblical miracles were literal and suggested that all people are part of God. In the Boston Daily Advertiser, Nathan Hale criticized Alcott's "flippant and off hand conversation" about serious topics from the Virgin birth of Jesus to circumcision. Joseph T. Buckingham called Alcott "either insane or half-witted" and "an ignorant and presuming charlatan". The book did not sell well; a Boston Lawyer bought 750 copies to use as waste paper.

Beginning in 1836, Alcott's membership in the Transcendental Club put him in such company as Ralph Waldo Emerson, Orestes Brownson and Theodore Parker. He became a member with the Club's second meeting and hosted their third. A biographer of Emerson described the group as "the occasional meetings of a changing body of liberal thinkers, agreeing in nothing but their liberality". Frederic Henry Hedge wrote of the group's nature: "There was no club in the strict sense... only occasional meetings of like-minded men and women". Alcott preferred the term "Symposium" for their group.

The temple school was widely denounced in the press. Reverend James Freeman Clarke was one of Alcott's few supporters and defended him against the harsh response from Boston periodicals. Alcott was rejected by most public opinion and, by the summer of 1837, he had only 11 students left and no assistant after Margaret Fuller moved to Providence, Rhode Island. The controversy had caused many parents to remove their children and, as the school closed, Alcott became increasingly financially desperate. Remaining steadfast to his pedagogy, a forerunner of progressive and democratic schooling, he alienated parents in a later "parlor school" by admitting an African American child to the class, whom he then refused to expel in the face of protests.

The couple's only son was born on April 6, 1839, but lived only a few minutes. The mother recorded: "Gave birth to a fine boy full grown perfectly formed but not living". It was in Germantown that the couple's second daughter was born. Louisa May Alcott was born on her father's birthday, November 29, 1832, at a half-hour past midnight. Bronson described her as "a very fine healthful child, much more so than Anna was at birth". The Germantown school, however, was faltering; soon only eight pupils remained. Their benefactor Haines died before Louisa's birth. He had helped recruit students and even paid tuition for some of them. As Abby wrote, his death "has prostrated all our hopes here". On April 10, 1833, the family moved to Philadelphia, where Alcott ran a day school. As usual, Alcott's methods were controversial; a former student later referred to him as "the most eccentric man who ever took on himself to train and form the youthful mind." Alcott began to believe Boston was the best place for his ideas to flourish. He contacted theologian william Ellery Channing for support. Channing approved of Alcott's methods and promised to help find students to enroll, including his daughter Mary. Channing also secured aid from Justice Lemuel Shaw and Boston mayor Josiah Quincy, Jr.

Like Emerson, Alcott was always optimistic, idealistic, and individualistic in thinking. Writer James Russell Lowell referred to Alcott in his poem "Studies for Two Heads" as "an angel with clipped wings". Even so, Emerson noted that Alcott's brilliant conversational ability did not translate into good writing. "When he sits down to write," Emerson wrote, "all his genius leaves him; he gives you the shells and throws away the kernel of his thought." His "Orphic Sayings", published in The Dial, became famous for their hilarity as dense, pretentious, and meaningless. In New York, for Example, The Knickerbocker published a parody titled "Gastric Sayings" in November 1840. A Writer for the Boston Post referred to Alcott's "Orphic Sayings" as "a train of fifteen railroad cars with one passenger".

With financial support from Emerson, Alcott left Concord on May 8, 1842, to a visit to England, leaving his brother Junius with his family. He met two admirers, Charles Lane and Henry C. Wright. The two men were Leaders of Alcott House, an experimental school based on Alcott's methods from the Temple School located about ten miles outside London. The school's founder, James Pierpont Greaves, had only recently died but Alcott was invited to stay there for a week. Alcott persuaded them to come to the United States with him; Lane and his son moved into the Alcott house and helped with family chores. Persuaded in part by Lane's abolitionist views, Alcott took a stand against the John Tyler administration's plan to annex Texas as a slave territory and refused to pay his poll tax. Abby May wrote in her journal on January 17, 1843, "A day of some excitement, as Mr. Alcott refused to pay his town tax... After waiting some time to be committed [to jail], he was told it was paid by a friend. Thus we were spared the affliction of his absence and the triumph of suffering for his principles." The annual poll tax was only $1.50. The incident inspired Henry David Thoreau, whose similar protest led to a night in jail and his essay "Civil Disobedience". Around this time, the Alcott family set up a sort of domestic post office to curb potential domestic tension. Abby May described her idea: "I thought it would afford a daily opportunity for the children, indeed all of us, to interchange thought and sentiment".

Lane and Alcott collaborated on a major expansion of their educational theories into a Utopian society. Alcott, however, was still in debt and could not purchase the land needed for their planned community. In a letter, Lane wrote, "I do not see anyone to act the money part but myself." In May 1843, he purchased a 90-acre (360,000 m) farm in Harvard, Massachusetts. Up front, he paid $1,500 of the total $1,800 value of the property; the rest was meant to be paid by the Alcotts over a two-year period. They moved to the farm on June 1 and optimistically named it "Fruitlands" despite only ten old apple trees on the property. In July, Alcott announced their plans in The Dial: "We have made an arrangement with the proprieter of an estate of about a hundred acres, which liberates this tract from human ownership".

In January 1844, Alcott moved his family to Still River, a village within Harvard but, on March 1, 1845, the family returned to Concord to live in a home they named "The Hillside" (later renamed "The Wayside" by Nathaniel Hawthorne). Both Emerson and Sam May assisted in securing the home for the Alcotts. While living in the home, Louisa began writing in earnest and was given her own room. She later said her years at the home "were the happiest years" of her life; many of the incidents in her novel Little Women (1868) are based on this period. Alcott renovated the property, moving a barn and painting the home a rusty olive color, as well as tending to over six acres of land. On May 23, 1845, Abby May was granted a sum from her father's estate which was put into a trust fund, granting minor financial security. That summer, Bronson Alcott let Henry David Thoreau borrow his ax to prepare his home at Walden Pond.

In 1848, Abby May insisted they leave Concord, which she called "cold, heartless, brainless, soulless". The Alcott family put The Hillside up for rent and moved to Boston. There, next door to Peabody's book store on West Street, Bronson Alcott hosted a series based on the "Conversations" model by Margaret Fuller called "A Course on the Conversations on Man—his History, Resources, and Expectations". Participants, both men and women, were charged three dollars to attend or five dollars for all seven lectures. In March 1853, Alcott was invited to teach fifteen students at Harvard Divinity School in an extracurricular, non-credit course.

Alcott and his family moved back to Concord after 1857, where he and his family lived in the Orchard House until 1877. In 1860, Alcott was named superintendent of Concord Schools.

Alcott voted in a presidential election for the first time in 1860. In his journal for November 6, 1860, he wrote: "At Town House, and cast my vote for Lincoln and the Republican candidates generally—the first vote I ever cast for a President and State officers." Alcott was an abolitionist and a friend of the more radical william Lloyd Garrison. He had attended a rally led by Wendell Phillips on behalf of 17-year-old Thomas Sims, a fugitive slave on trial in Boston. Alcott was one of several who attempted to storm the courthouse; when gunshots were heard, he was the only one who stood his ground, though the effort was unsuccessful. He had also stood his ground in a protest against the trial of Anthony Burns. A group had broken down the door of the Boston courthouse but guards beat them back. Alcott stood forward and asked the leader of the group, Thomas Wentworth Higginson, "Why are we not within?" He then walked calmly into the courthouse, was threatened with a gun, and turned back, "but without hastening a step", according to Higginson.

Henry David Thoreau died on May 6, 1862, likely from an illness he caught from Alcott two years earlier. At Emerson's request, Alcott helped arrange Thoreau's funeral, which was held at First Parish Sanctuary in Concord, despite Thoreau having disavowed membership in the church when he was in his early twenties. Emerson wrote a eulogy, and Alcott helped plan the preparations. Only two years later, neighbor Nathaniel Hawthorne died as well. Alcott served as a pallbearer along with Louis Agassiz, James Thomas Fields, Oliver Wendell Holmes, Sr., Henry Wadsworth Longfellow, and others. With Hawthorne's death, Alcott worried that few of the Concord notables remained. He recorded in his journal: "Fair figures one by one are fading from sight." The next year, Lincoln was assassinated, which Alcott called "appalling news".

In 1862, Louisa moved to Washington, D.C. to volunteer as a nurse. On January 14, 1863, the Alcotts received a telegram that Louisa was sick; Bronson immediately went to bring her home, briefly meeting Abraham Lincoln while there. Louisa turned her experience into the book Hospital Sketches. Her father wrote of it, "I see nothing in the way of a good appreciation of Louisa's merits as a woman and a Writer."

Alcott's published books, all from late in his life, include Tablets (1868), Concord Days (1872), New Connecticut (1881), and Sonnets and Canzonets (1882). Louisa May attended to her father's needs in his final years. She purchased a house for her sister Anna which had been the last home of Henry David Thoreau, now known as the Thoreau-Alcott House. Louisa and her parents moved in with Anna as well.

The members of the Alcott family were not happy with their Fruitlands experience. At one point, Abby May threatened that she and their daughters would move elsewhere, leaving Bronson behind. Louisa May Alcott, who was ten years old at the time, later wrote of the experience in Transcendental Wild Oats (1873): "The band of brothers began by spading garden and field; but a few days of it lessened their ardor amazingly."

After the death of his wife Abby May on November 25, 1877, Alcott never returned to Orchard House, too heartbroken to live there. He and Louisa May collaborated on a memoir and went over her papers, letters, and journals. "My heart bleeds with the memories of those days", he wrote, "and even long years, of cheerless anxiety and hopeless dependence." Louisa noted her father had become "restless with his anchor gone". They gave up on the memoir project and Louisa burned many of her mother's papers.

On January 19, 1879, Alcott and Franklin Benjamin Sanborn wrote a prospectus for a new school which they distributed to potentially interested people throughout the country. The result was the Concord School of Philosophy and Literature, which held its first session in 1879 in Alcott's study in the Orchard House. In 1880 the school moved to the Hillside Chapel, a building next to the house, where he held conversations and, over the course of successive summers, as he entered his eighties, invited others to give lectures on themes in philosophy, religion and letters. The school, considered one of the first formal adult education centers in America, was also attended by foreign scholars. It continued for nine years.

In April 1882, Alcott's friend and benefactor Ralph Waldo Emerson was sick and bedridden. After visiting him, Alcott wrote, "Concord will be shorn of its human splendor when he withdraws behind the cloud." Emerson died the next day. Alcott himself moved out of Concord for his final years, settling at 10 Louisburg Square in Boston beginning in 1885.

The Concord School of Philosophy, which closed following Alcott's death in 1888, was reopened almost 90 years later in the 1970s. It has continued functioning with a Summer Conversational Series in its original building at Orchard House, now run by the Louisa May Alcott Memorial Association.

The Alcotts hosted a steady stream of visitors at The Hillside, including fugitive slaves, which they hosted in secret as a station of the Underground Railroad. Alcott's opposition to slavery also fueled his opposition to the Mexican–American War which began in 1846. He considered the war a blatant attempt to extend slavery and asked if the country was made up of "a people bent on conquest, on getting the golden treasures of Mexico into our hands, and of subjugating foreign peoples?"

Modern critics often fault Alcott for not being able to financially support his family. Alcott himself worried about his own prospects as a young man, once writing to his mother that he was "still at my old trade—hoping." Alcott held his principles above his well-being. Shortly before his marriage, for Example, his Future father-in-law Colonel Joseph May helped him find a job teaching at a school in Boston run by the Society of Free Enquirers, followers of Robert Owen, for a lucrative $1,000 to $1,200 annual salary. He refused it because he did not agree with their beliefs, writing, "I shall have nothing to do with them."