Two years into his apprenticeship Booth was converted to Methodism. He then read extensively and trained himself in writing and in speech, becoming a Methodist local preacher. Booth was encouraged to be an evangelist primarily through his best friend, Will Sansom. Sansom and Booth both began in the 1840s to preach to the poor and the sinners of Nottingham, and Booth would probably have remained as Sansom's partner in his new Mission ministry, as Sansom titled it, if Sansom had not died of tuberculosis, in 1849.

William Booth was born in Sneinton, Nottingham, the second son of five children born to Samuel Booth and his second wife, Mary Moss. Booth's father was relatively wealthy by the standards of the time, but during William's childhood, the family descended into poverty. In 1842, Samuel Booth, who could no longer afford his son's school fees, apprenticed the 13-year-old william Booth to a pawnbroker. Samuel Booth died on 23 September 1842.

When his apprenticeship ended in 1848, Booth was unemployed and spent a year looking in vain for work. In 1849, Booth reluctantly left his family and moved to London, where he again found work with a pawnbroker. Booth tried to continue lay preaching in London, but the small amount of preaching work that came his way frustrated him, and so he resigned as a lay preacher and took to open-air evangelising in the streets and on Kennington Common.

In 1851, Booth joined the Reformers (Methodist Reform Church), and on 10 April 1852, his 23rd birthday, he left pawnbroking and became a full-time preacher at their headquarters at Binfield Chapel in Clapham. william styled his preaching after the revivalist American James Caughey, who had made frequent visits to England and preached at the church in Nottingham where Booth was a member, Broad Street Chapel. Just over a month after he started full-time preaching, on 15 May 1852, william Booth became formally engaged to Catherine Mumford.

William Booth and Catherine Mumford were married on 17 June 1855 at Stockwell New Chapel, at that time part of Surrey. They had eight children:

Though Booth became a prominent Methodist evangelist, he was unhappy that the annual conference of the denomination kept assigning him to a pastorate, the duties of which he had to neglect to respond to the frequent requests that he do evangelistic campaigns. At the Liverpool conference in 1861, after having spent three years at Gateshead, his request to be freed for evangelism full-time was refused yet again, and Booth resigned from the ministry of the Methodist New Connexion.

Having been founded as the East London Christian Mission in 1865, the name The Salvation Army developed from an incident in May 1878. william Booth was dictating a letter to his secretary George Scott Railton and said, "We are a volunteer army." Bramwell Booth heard his father and said, "Volunteer, I'm no volunteer, I'm a regular!" Railton was instructed to cross out the word "volunteer" and substitute the word "salvation".(The printer's proof copy of the Missions' report for 1878 declared "The Christian Mission Is A Volunteer Army", but the corrected proof read "The Christian Mission Is ... A Salvation Army") The Salvation Army was modelled after the military, with its own flag (or colours) and its own music, often with Christian words to popular and folkloric tunes sung in the pubs. Booth and the other Soldiers in "God's Army" would wear the army's own uniform, 'putting on the armour,' for meetings and ministry work. He became the "General" and his other ministers were given appropriate ranks as "officers". Other members became "soldiers".

During its early years The Salvation Army faced a great deal of opposition, especially from those in the alcohol-selling industry who were concerned that the activities of Booth and his followers would persuade the poorer classes to stop drinking. One group opposed to Booth and The Salvation Army was the Skeleton Army, a diffuse group, particularly in Southern England, that opposed and disrupted The Salvation Army's marches against alcohol from the early 1880s until about 1892. Clashes between the two groups lead to the deaths of several Salvationists and injuries to many others. During 1882 alone 662 Salvation Army Soldiers were assaulted: 251 of them were women and 23 of them were under fifteen years of age.

Booth regularly published a magazine and was the author of a number of books; he also composed several songs. His book In Darkest England and the Way Out not only became a best-seller after its 1890 release, it set the foundation for the army's modern social welfare approach. It compared what was considered "civilised" England with "Darkest Africa" – a land then considered poor and backward. What Booth suggested was that much of London and greater England after the Industrial Revolution was not better off in the quality of life than those in the underdeveloped world.

In 1899, Booth suffered from blindness in both eyes, but with a short rest, was able to recover his sight. In 1904 he took part in a "motorcade" when he was driven around Great Britain, stopping off in cities, towns and villages to preach to the assembled crowds from his open-top car. In 1906 Booth was made a Freeman of the City of London, and was granted an honorary degree from the University of Oxford. In 1902 he was invited to attend the coronation of King Edward VII.

His last visit to the United States was made in 1907, and in 1909 he embarked on a six-month motor tour of the United Kingdom. During this tour he discovered he was blind in his right eye and the sight in his left eye was dimmed by cataracts. The rest of the tour had to be cancelled. On 21 August 1909 a surgeon at Guy's Hospital removed his right eye. Despite this setback, in 1910 Booth campaigned in the Netherlands, Denmark, Germany, Switzerland and Italy. On his return to England he embarked on his seventh and last motor tour.

William Booth was 83 years old when he died (or, in Salvationist parlance, was Promoted to Glory) at his home in Hadley Wood, London. At the three-day lying in state at Clapton Congress Hall 150,000 people filed past his casket. On 27 August 1912 Booth's funeral Service was held at London’s Olympia where 40,000 people attended, including Queen Mary, who sat almost unrecognised far to the rear of the great hall.

Statues of each of the Booths by George Edward Wade were erected on Champion Hill in London, next to the Salvation Army's training college in London in 1929. Replicas of these statues stand in the Mile End Road, close to the site of the first Salvation Army meeting: that of william was unveiled in 1979, and that of Catherine in 2015.

In Booth's honour, Vachel Lindsay wrote the poem, "General william Booth Enters Into Heaven". Charles Ives, who had been Evangeline Booth's neighbour, set the poem to music. In 1990 a diesel locomotive in the British Rail fleet was named 'The william Booth'.

In Darkest England and the Way Out was reprinted several times and lately in 2006.

The following day Booth's funeral procession set out from International Headquarters. As it moved off 10,000 uniformed Salvationists fell in behind. Forty Salvation Army bands played the "Dead March" from Handel's Saul as the vast procession set off. He was buried with his wife Catherine Booth in the main London burial ground for 19th century non-conformist ministers and tutors, the non-denominational Abney Park Cemetery in Stoke Newington.

There are also other works that have focused on the impact and significance of In Darkest England. For Example, marking the 125th anniversary of the publication of In Darkest England, the book Darkness and Deliverance: 125 Years of the Darkest England Scheme contains fifteen chapters from leading and emerging authors that explore various historical aspects and Future implications of the Darkest England scheme.

He proposed a strategy to apply the Christian Gospel and work ethic to the problems. The book speaks of abolishing vice and poverty by establishing homes for the homeless, farm communities such as Hadleigh Farm where the urban poor can be trained in agriculture, training centres for prospective emigrants, homes for fallen women and released prisoners, aid for the poor, and help for drunkards. He also lays down schemes for poor men’s lawyers, banks, clinics, industrial schools and even a seaside resort. He says that if the state fails to meet its social obligations it will be the task of each Christian to step into the breach. However, Booth was not departing from his spiritual convictions to set up a socialist or communist society or sub-class, supported by people forced to Finance his plans; Booth's ultimate aim was to get people "saved."