Barton was born on December 25, 1821, in North Oxford, Massachusetts. Barton's father was Captain Stephen Barton, a member of the local militia and a selectman who inspired his daughter with patriotism and a broad humanitarian interest. He was a soldier under the command of General Anthony Wayne in his crusade against the Indians in the North West. He was also the leader of progressive thought in the Oxford village area. Barton's mother was Sarah Stone Barton.

Clara Barton became an educator in 1838 for 12 years in schools in Canada and West Georgia. Barton fared well as a Teacher and knew how to handle rambunctious children, particularly the boys, since as a child she enjoyed her male cousins' and brothers' company. She learned how to act like them, making it easier for her to relate to and control the boys in her classroom since they respected her. After her mother's death in 1851, the family home closed down. Barton decided to further her education by pursuing writing and languages at the Clinton Liberal Institute in New York. In this college town, she developed many friendships that broadened her point of view on many issues concurring at the time. The principal of the institute recognized her tremendous abilities and admired her work. This friendship lasted for many years, eventually turning into a romance. As a Writer, her terminology was pristine and easy to understand. Her writings and bodies of work could instruct the local statesmen. No one could exceed her outstanding Service to humanity in war and in peace.

To assist Clara with overcoming her shyness, her parents persuaded her to become a schoolteacher. She achieved her first teacher's certificate in 1839, at only 17 years old. This profession interested Barton greatly and helped motivate her; she ended up conducting an effective redistricting campaign that allowed the children of workers to receive an education. Successful projects such as this gave Clara the confidence needed when she demanded equal pay for teaching.

While teaching in Hightstown, Clara Barton learned about the lack of public schools in Bordentown, the neighboring city. In 1852, she was contracted to open a free school in Bordentown, which was the first ever free school in New Jersey. She was successful, and after a year she had hired another woman to help teach over 600 people. Both women were making $250 a year. This accomplishment compelled the town to raise nearly $4,000 for a new school building. Once completed, though, Barton was replaced as principal by a man elected by the school board. They saw the position as head of a large institution to be unfitting for a woman. She was demoted to "female assistant" and worked in a harsh environment until she had a nervous breakdown along with other health ailments, and quit.

In 1855, she moved to Washington D.C. and began work as a clerk in the US Patent Office; this was the first time a woman had received a substantial clerkship in the federal government and at a salary equal to a man's salary. For three years, she received much abuse and slander from male clerks. Subsequently, under political opposition to women working in government offices, her position was reduced to that of copyist, and in 1856, under the administration of James Buchanan, she was fired because of her "Black Republicanism." After the election of Abraham Lincoln, having lived with relatives and friends in Massachusetts for three years, she returned to the patent office in the autumn of 1861, now as temporary copyist, in the hope she could make way for more women in government Service.

On April 19, 1861, the Baltimore Riot resulted in the first bloodshed of the American Civil War. Victims within the Massachusetts regiment were transported to Washington D.C. after the violence, which happened to be Clara Barton's home at the time. Wanting to serve her country, Barton went to the railroad station when the victims arrived and nursed 40 men. Barton provided crucial, personal assistance to the men in uniform, many of whom were wounded, hungry and without any supplies besides what they carried on their backs. She began helping them by personally taking supplies to the unfinished Capitol Building where the young men of the 6th Massachusetts Militia, who had been attacked in Baltimore, Maryland, were housed.

It was on that day that she identified herself with army work and began her efforts towards collecting medical supplies for the Union Soldiers. Prior to distributing provisions directly onto the battlefield and gaining further support, Barton used her own living quarters as a storeroom and distributed supplies with the help of a few friends in early 1862, despite opposition in the War Department and among field Surgeons. Ladies' Aid societies helped in sending bandages, food, and clothing that would later be distributed during the Civil War. In August 1862, Barton finally gained permission from Quartermaster Daniel Rucker to work on the front lines. She gained support from other people who believed in her cause. These people became her patrons, her most supportive being Senator Henry Wilson of Massachusetts.

In 1863 she began a romantic relationship with an officer, Colonel John J. Elwell.

In 1864, she was appointed by Union General Benjamin Butler as the "lady in charge" of the hospitals at the front of the Army of the James. Among her more harrowing experiences was an incident in which a bullet tore through the sleeve of her dress without striking her and killed a man to whom she was tending. She was known as the "American Nightingale." She was also known as the "Angel of the Battlefield" after she came to the aid of the overwhelmed surgeon on duty following the battle of Cedar Mountain in Northern Virginia in August 1862. She arrived at a field hospital at midnight with a large amount of supplies to help the severely wounded Soldiers. This naming came from her frequent timely assistance as she served troops at the battles of Fairfax Station, Chantilly, Harpers Ferry, South Mountain, Antietam, Fredericksburg, Charleston, Petersburg and Cold Harbor.

Barton achieved widespread recognition by delivering lectures around the country about her war experiences in 1865–1868. During this time she met Susan B. Anthony and began a long association with the woman's suffrage movement. She also became acquainted with Frederick Douglass and became an Activist for civil rights. After her country wide tour she was both mentally and physically exhausted and under doctor's orders to go somewhere that would take her far from her current work. She closed the Missing Soldiers Office in 1868 and traveled to Europe. In 1869, during her trip to Geneva, Switzerland, Barton was introduced to the Red Cross and Dr. Appia; who later would invite her to be the representative for the American branch of the Red Cross and even help her find financial beneficiaries for the start of the American Red Cross. She was also introduced to Henry Dunant's book A Memory of Solferino, which called for the formation of national societies to provide relief voluntarily on a neutral basis.

At the beginning of the Franco-Prussian War, in 1870, she assisted the Grand Duchess of Baden in the preparation of military hospitals, and gave the Red Cross society much aid during the war. At the joint request of the German authorities and the Strasbourg Comité de Secours, she superintended the supplying of work to the poor of Strasbourg in 1871, after the Siege of Paris, and in 1871 had charge of the public distribution of supplies to the destitute people of Paris. At the close of the war, she received honorable decorations of the Golden Cross of Baden and the Prussian Iron Cross.

When Barton returned to the United States, she inaugurated a movement to gain recognition for the International Committee of the Red Cross (ICRC) by the United States government. In 1873, she began work on this project. In 1878, she met with President Rutherford B. Hayes, who expressed the opinion of most Americans at that time which was the U.S. would never again face a calamity like the Civil War. Barton finally succeeded during the administration of President Chester Arthur, using the argument that the new American Red Cross could respond to crises other than war such as natural disasters like earthquakes, forest fires, and hurricanes.

Barton became President of the American branch of the society, which held its first official meeting at her I Street apartment in Washington, DC, May 21, 1881. The first local society was founded August 22, 1882 in Dansville, Livingston County, New York, where she maintained a country home.

Domestically in 1884 she helped in the floods on the Ohio river, provided Texas with food and supplies during the famine of 1887 and took workers to Illinois in 1888 after a tornado and that same year to Florida for the yellow fever epidemic. Within days after the Johnstown Flood in 1889, she led her delegation of 50 doctors and Nurses in response. In 1897, responding to the humanitarian crisis in the Ottoman Empire in the aftermath of the Hamidian Massacres, Barton sailed to Constantinople and after long negotiations with Abdul Hamid II, opened the first American International Red Cross headquarters in the heart of Turkey. Barton herself traveled along with five other Red Cross expeditions to the Armenian provinces in the spring of 1896, providing relief and humanitarian aid. Barton also worked in hospitals in Cuba in 1898 at the age of seventy-seven. Barton's last field operation as President of the American Red Cross was helping victims of the Galveston hurricane in 1900. The operation established an orphanage for children.

As criticism arose of her mixing professional and personal resources, Barton was forced to resign as President of the American Red Cross in 1904, at the age of 83 because of her egocentric leadership style fitting poorly into the formal structure of organizational charity. She had been forced out of office by a new generation of all-male scientific experts who reflected the realistic efficiency of the Progressive Era rather than her idealistic humanitarianism. In memory of the courageous women of the civil war, the Red Cross Headquarters was founded. During the dedication, not one person said a word. This was done in order to honor the women and their services. After resigning, Barton founded the National First Aid Society.

Although not formally a member of the Universalist Church of America, in a 1905 letter to the widow of Carl Norman Thrasher, she identified herself with her parents' church as a "Universalist". While she was not an active member of her parents' church, Clara wrote about how well known her family was in her hometown and how many relationships her father formed with others in their town through their church and religion.

She continued to live in her Glen Echo, Maryland home which also served as the Red Cross Headquarters upon her arrival to the house in 1897. Barton published her autobiography in 1907, titled The Story of My Childhood. On April 12, 1912 at the age of 90, she died in her home. The cause of death was pneumonia.

In 1975, the Clara Barton National Historic Site, located at 5801 Oxford Road, Glen Echo, Maryland, was established as a unit of the National Park Service at Barton's home, where she spent the last 15 years of her life. As the first National Historic Site dedicated to the accomplishments of a woman, it preserves the early history of the American Red Cross, since the home also served as an early headquarters of the organization. The North Oxford, Massachusetts, house in which she was born is now also a museum.

In 1997, General Services Administration carpenter Richard Lyons was hired to check out the building for its demolition. He found a treasure trove of Clara Barton items in the attic, including signs, clothing, Civil War soldier's socks, an army tent, Civil War-era newspapers, and many documents relating to the Office of Missing Soldiers. This discovery led to the NPS saving the building from demolition. It took years, however, for the site to be restored. The Clara Barton's Missing Soldiers Office Museum, run by the National Museum of Civil War Medicine, opened in 2015.

The society's role changed with the advent of the Spanish–American War during which it aided refugees and prisoners of the civil war. Once the Spanish–American War was over the great people of Santiago built a statue in honor of Clara Barton in the town square. The statue of Clara Barton still stands there today.