He was born in London in 1923. He was the eldest son of Sir Oswald Mosley, 6th Baronet, a British politician, and his first wife, Lady Cynthia Mosley, a daughter of The 1st Marquess Curzon of Kedleston (Viceroy of India and Foreign Secretary). In 1932 his father, Sir Oswald Mosley, founded the British Union of Fascists and became an open supporter of Benito Mussolini. In 1933, when he was only 9, Nicholas's mother, Lady Cynthia, died and in 1936 Diana Mitford, one of the Mitford sisters, who was already his father's mistress, became his stepmother.

As a young boy, he began to stammer and attended weekly sessions with the speech therapist Lionel Logue to help him to overcome his disorder. He later said that his father claimed never really to have noticed this stammer, but still, he may, as a result of it, have been less aggressive when speaking to him than towards other people. Mosley was educated at Eton and Balliol College, Oxford. In 1940, his father was interned because of his campaigning against the war with Germany. The younger Mosley was still soon commissioned into the Rifle Brigade and saw active service in Italy, winning the Military Cross in 1945.

In 1966, Mosley succeeded his aunt Irene Curzon, 2nd Baroness Ravensdale, his mother's elder sister, as Baron Ravensdale, thus gaining a seat in the House of Lords. On the death of his father, on 3 December 1980, he also succeeded to his father's baronetcy. In 1983, after his father's death, Lord Ravensdale published Beyond the Pale: Sir Oswald Mosley and Family 1933–1980, in which he proved to be a harsh critic of his father. He called into question his father's motives and even Oswald's understanding of politics. The book contributed to the Channel 4 television programme Mosley (1998), based on Oswald Mosley's life. At the end of the serial, Nicholas is portrayed meeting his father in prison to ask him about his national allegiance.