Boswell was born near St Giles Cathedral in Edinburgh on 29 October 1740. He was the eldest son of a judge, Alexander Boswell, 8th Laird of Auchinleck and his wife Euphemia Erskine; he inherited his father’s estate Auchinleck in Ayrshire. Boswell's mother was a strict Calvinist, and he felt that his father was cold to him. As a child, he was delicate and suffered from some type of nervous ailment which appeared to be inherent and would afflict him sporadically all through his life. At the age of five, he was sent to James Mundell's academy, an advanced institution by the standards of the time, where he was instructed in English, Latin, writing and arithmetic. Boswell was unhappy there, and his sickliness began to manifest itself in the physical indicants associated with night fears and extreme shyness.

In view of this, the now-eight-year-old was removed from the academy and educated by a string of private tutors who included John Dunn and a Mr Fergusson. The former had rather more success than his successor: he versed his charge in the joys of literature (not least of all the Spectator essays) and opened his eyes to the pleasances of religion. Dunn was also present during, if not directly involved in, Boswell's serious affliction of 1752, when he was rusticated to the hamlet of Moffat in northern Dumfriesshire. This afforded him his first experience of genuine society, and his recovery was rapid and complete. It may, however, have inculcated the notion that travel and entertainment were his best sedatives.

At thirteen, Boswell was enrolled into the arts course at the University of Edinburgh, studying there from 1753 to 1758. Midway through his studies, he suffered a serious depression and nervous illness, but, when he recovered, he had thrown off all signs of delicacy and attained robust health. Boswell had swarthy skin, black hair and dark eyes; he was of average height, and he tended to plumpness. His appearance was alert and masculine, and he had an ingratiating sense of good humour.

Upon turning nineteen, he was sent to continue his studies at the University of Glasgow, where he was taught by Adam Smith. While at Glasgow, Boswell decided to convert to Catholicism and become a monk. Upon learning of this, his father ordered him home. Instead of obeying, though, Boswell ran away to London, where he spent three months, living the life of a libertine, before he was taken back to Scotland by his father. Upon returning, he was re-enrolled at Edinburgh University and forced by his father to sign away most of his inheritance in return for an allowance of £100 a year.

On 30 July 1762, Boswell took his oral law exam, which he passed with some skill. Upon this success, Lord Auchinleck decided to raise his son's allowance to £200 a year and allowed him to return to London. It was during his second spell there that Boswell wrote his London Journal and, on 16 May 1763, met Johnson for the first time. The pair became friends almost immediately. Johnson eventually nicknamed him "Bozzy".

The first conversation between Johnson and Boswell is quoted in The Life of Samuel Johnson as follows:

[Boswell:] "Mr. Johnson, I do indeed come from Scotland, but I cannot help it."[Johnson:] "That, Sir, I find, is what a very great many of your countrymen cannot help."

It is widely believed that Johnson despised the Scots; however, on being specifically asked the question, he admitted that this prejudice was without basis.

It was around three months after this first encounter with Johnson that Boswell departed for Europe with the initial goal of continuing his law studies at Utrecht University. He spent a year there and although desperately unhappy the first few months, eventually quite enjoyed his time in Utrecht. He befriended and fell in love with Isabelle de Charrière, also known as Belle van Zuylen, a vivacious young Dutchwoman of unorthodox opinions, his social and intellectual superior. Boswell admired the young widow Geelvinck who refused to marry him. After this, Boswell spent most of the next two years travelling around the continent. During this time he met Jean-Jacques Rousseau and made a pilgrimage to Rome. Boswell also travelled to Corsica to meet one of his heroes, the independence leader Pasquale Paoli. His well-observed diaries of this time have been compiled into two books Boswell in Holland and Boswell and the Grand Tour.

Boswell returned to London in February 1766 accompanied by Rousseau's mistress, with whom he may have had a brief affair on the journey home. After spending a few weeks in the capital, he returned to Scotland to take his final law exam. He passed the exam and became an advocate. He practised for over a decade, during which time he spent no more than a month every year with Johnson. Nevertheless, he returned to London annually to mingle with Johnson and the rest of the London literary crowd, and to escape his mundane existence in Scotland. He found enjoyment in playing the intellectual rhyming game crambo with his peers.

Some of his journal entries and letters from this period describe his amatory exploits. Thus, in 1767, in a letter to W.J.Temple, he wrote, "I got myself quite intoxicated, went to a Bawdy-house and past a whole night in the arms of a Whore. She indeed was a fine strong spirited Girl, a Whore worthy of Boswell if Boswell must have a whore" A few years earlier, he wrote that during a night with an actress named Louisa "five times was I fairly lost in supreme rapture. Louisa was madly fond of me; she declared I was a prodigy and asked me if this was not extraordinary for human nature." Though he sometimes used a condom for protection, he contracted venereal disease at least seventeen times

Boswell married his cousin, Margaret Montgomerie, in November 1769. She remained faithful to Boswell, despite his frequent liaisons with prostitutes, until her death of tuberculosis in 1789. After his infidelities, he would deliver tearful apologies to her and beg her forgiveness, before again promising her, and himself, that he would reform. James and Margaret had four sons and three daughters. Two sons died in infancy; the other two were Alexander (1775—1822) and James (1778—1822). Their daughters were Veronica (1773—1795), Euphemia (1774-ca. 1834) and Elizabeth (1780—1814). Boswell also had at least two illegitimate children, Charles (1762—1764) and Sally (1767-1768?).

Despite his relative literary success with accounts of his European travels, Boswell was an unsuccessful advocate, with the exception of the copyright infringement case of Donaldson v Beckett where Boswell was the advocate of the Scottish bookseller, Alexander Donaldson. By the late 1770s, Boswell descended further and further into alcoholism and gambling addiction. Throughout his life, from childhood until death, he was beset by severe swings of mood. His depressions frequently encouraged, and were exacerbated by, his various vices. His happier periods usually saw him relatively vice-free. His character mixed a superficial Enlightenment sensibility for reason and taste with a genuine and somewhat romantic love of the sublime and a propensity for occasionally puerile whimsy. The latter, along with his tendency for drink and other vices, caused many contemporaries and later observers to regard him as being too lightweight to be an equal in the literary crowd that he wanted to be a part of. However, his humour and innocent good nature won him many lifelong friends.

Boswell was a frequent guest of Lord Monboddo at Monboddo House, a setting where he gathered significant observations for his writings by association with Samuel Johnson, Robert Burns, Lord Kames, Lord Monboddo and other luminaries.

After Johnson's death in 1784, Boswell moved to London to try his luck at the English Bar, which proved even more unsuccessful than his career in Scotland. He also offered to stand for Parliament but failed to get the necessary support, and he spent the final years of his life writing his Life of Johnson. During this time his health began to fail due to venereal disease and his years of drinking. Boswell died in London in 1795.

When the Life of Johnson was published in 1791 it at once commanded the admiration that Boswell had sought for so long, and it has suffered no diminution since. Its style was revolutionary ... unlike other biographies of that era it directly incorporated conversations that Boswell had noted down at the time for his journals. He also included far more personal and human details than contemporary readers were accustomed to. Instead of writing a respectful and dry record of Johnson's public life, in the style of the time, he painted a vivid portrait of the complete man. It is still often said to be the greatest biography yet written, and the longevity of Johnson's fame perhaps owes much to the work.

It has often been asked how a man such as Boswell could have produced so remarkable a work as the Life of Johnson. Among those who attempted an answer were Macaulay and Carlyle: the former argued that Boswell's uninhibited folly and triviality were his greatest qualifications; the latter replied that beneath such traits were a mind to discern excellence and a heart to appreciate it, aided by the power of accurate observation and considerable dramatic ability.(Macaulay's venomous condemnation of Boswell's personality may have had a political foundation: Boswell was a Tory, and as such a target for Whig historian Macaulay's attacks. In addition, Macaulay's grandfather was the victim of one of Johnson's sharpest rebukes: "Sir, are you so grossly ignorant of human nature, as not to know that a man may be very sincere in good principles, without having good practice?"). A Short Biographical Dictionary of English Literature by John W. Cousin - Full Text Free Book (Part 2/13)

Boswell was present at the meeting of the Committee for the Abolition of the Slave Trade in May 1787 set up to persuade William Wilberforce to lead the abolition movement in Parliament. However, the abolitionist Thomas Clarkson records that by 1788 Boswell "after having supported the cause... became inimical to it."

Boswell's most prominent display of support for the slavery movement was his 1791 poem 'No Abolition of Slavery; or the Universal Empire of Love,' which lampooned Clarkson, Wilberforce and Pitt. The poem also supports the common suggestion of the pro-slavery movement, that the slaves actually enjoyed their lot: "The cheerful gang! - the negroes see / Perform the task of industry."

In the 1920s a great part of Boswell's private papers, including intimate journals for much of his life, were discovered at Malahide Castle, north of Dublin. These provide a hugely revealing insight into the life and thoughts of the man. They were sold to the American collector Ralph H. Isham and have since passed to Yale University, which has published general and scholarly editions of his journals and correspondence. A second cache was discovered soon after and also purchased by Isham. A substantially longer edition of A Journal of a Tour to the Hebrides was published in 1936 based on his original manuscript. His London Journal 1762-63, the first of the Yale journal publications, appeared in 1950. The last, The Great Biographer, 1789-1795, was published in 1989.

These detailed and frank journals include voluminous notes on the grand tour of Europe that he took as a young man and, subsequently, of his tour of Scotland with Johnson. His journals also record meetings and conversations with eminent individuals belonging to The Club, including Lord Monboddo, David Garrick, Edmund Burke, Joshua Reynolds and Oliver Goldsmith.