He had a privileged upbringing but his parents separated and he was brought up by his mother and paternal Grandfather, the fourth Baronet Mosley. He was always called 'Tom' by family and friends.
He was educated at Winchester College and the Royal Military College at Sandhurst. During World War I, he was commissioned in the 16th Lancers and fought on the Western Front. He transferred to the Royal Flying Corps but a crash left him with a permanent limp. He returned to the trenches before the injury was fully healed and, at the Battle of Loos he passed out at his post from the pain. He was assigned to desk jobs for the rest of the war.
Mosley became an Conservative MP for Harrow in 1918, the youngest member of the British House of Commons. He soon distinguished himself as an orator and political player, albeit marked with extreme self-confidence.
He resigned from the Conservatives in 1922 as a consequence of his disagreement with the government over the Irish policy and their use of the Black and Tans to suppress the Irish population.
He retained his seat as an Independent MP for Harrow (1922-24). He then switched to Labour (1924 and 1929-31), being rewarded with the post of Chancellor of the Duchy of Lancaster in 1929. He resigned in 1930 following the rejection of his radical economic policy ideas. He quickly founded the radical New Party but when that failed, following meetings with Mussolini and other Fascists, he created the British Union of Fascists (BUF) in 1932.
The British Union of Fascists was a union of numerous smaller extreme nationalist parties, Mosley instituted a black uniform, gaining the party the nickname blackshirts. The BUF was anti-Communist and protectionist, claimed membership was as high as 50,000, and the Daily Mail was an early supporter. Among his followers were the novelist Henry Williamson, and military theorist J.F.C. Fuller.
The party became the victim of Communist and Jewish violence especially in London. The government was sufficiently concerned to pass the Public Order Act of 1936, which was intended to destroy the movement but failed.
In the London County Council elections in 1937 the Movement scored high votes in its east London strongholds. The BUF was completely banned in May 1940 and Mosley and 740 other senior Fascists were interned for much of WW II. Mosley was released in 1943.
After the war he made a number of attempts to return to politics (1947, 1959, 1966), but never successfully. He became noted for his advocacy of Britain's entry into the European Economic Community.
He had three children with his first wife. He and the second Lady Mosley had two sons. A noted philanderer, Mosley had numerous affairs, during his first marriage to Lady Cynthia, with her sister Lady Alexandra Metcalfe as well as her stepmother, Lady Curzon.