Composer, music publisher, and pianist. As a boy he studied counterpoint with Antonio Burom, organ with Condiceli, and possibly voice with Giuseppe Carpani. 
An Englishman, Peter Beckford, heard him play in 1766 and paid Clementi's father to allow him to take the boy to his English country estate for intensive musical studies, and to serve as his house musician. For the next seven years Clementi took advantage of the relatively isolated Dorsetshire setting, practicing harpsichord and producing his first compositions, a Mass and an oratorio (both lost), and keyboard sonatas. At the end of this period he moved to London, began concertizing perhaps in 1775, and in 1779 and 1780 published his opp. 2-4 (works for keyboard). Among positions he held during the 1770s was that of keyboard conductor at the King's Theatre, Haymarket. In 1780 he began a concert tour that included Paris (where he allegedly played for Marie Antoinette) and Vienna, where he engaged in the famous keyboard contest with Mozart in 1781. 
The latter wrote to his father of Clementi's remarkable technique, especially in passages of thirds or octaves. "Otherwise he hasn't a kreutzer's worth of feeling or taste. In a word--a sheer mechanicus" (12 Jan. 1782). In addition, Clementi's opp. 7, 9, and 10 were published in Vienna, 1782-83. He returned to London in 1783 by way of Lyons, where his op. 8 was published (three sonatas for keyboard). After reestablishing London's Hanover Square Concerts in 1783, he returned to Lyons for the sake of an 18-year-old girl, whose disapproving father again drove him off.
During the period 1784 to 1790 he published (as opp. 11-25) some 40 works for keyboard. He settled permanently in London, becoming a successful pianist, pedagogue, and music publisher; be continued to concertize, and his firm (originally a series of partnerships in which Longman and Clementi were name partners) thrived. His pupils included Johann Baptist Cramer. 
When his popularity in London began declining in the 1790s, he concentrated on teaching and publishing. His Introduction to the Art of Playing on the Piano Forte appeared in London in 1801. Nevertheless he continued composing, working steadily during the 1790s on a set of six grand symphonies (never completed) that he hoped would secure his place in music history. In 1802 he embarked on another concert tour, to Paris, Vienna, and St. Petersburg, with his pupil John Field. While in Vienna in 1807, Clementi the publisher acquired rights to issue five of Beethoven's latest compositions. Back in London, he was appointed director of the Philharmonic Society. 
During the 1820s he continued directing concerts of his works and made other tours of the Continent. His three big piano sonatas op. 50 appeared in 1821 in London, Paris, and Leipzig. The three volumes of his highly successful pedagogical work Gradus ad Parnassum, or The Art of Playing on the Piano Forte appeared in 1817, 1819, and 1826. He died two years after his retirement from publishing in 1830. His musical legacy includes, in addition to the more than 100 piano sonatas (many accompanied), many works for 4-hand piano, two completed symphonies, four incomplete symphonies (two completed by Alfredo Casella), a Piano Concerto in C (1796), and many smaller works for piano.