Leslie Crombie was born in York on 10 June 1923, the second eldest, and only boy, of Walter Crombie and Gladys (née Clarkson). On his father's side his great-grandfather had kept a tobacconist shop in York and his grandfather, George, had founded a prosperous legal practice in the City of York. On his mother's side, Leslie's great–grandfather originated from London and settled in York after helping to build the York Railway Station. Leslie's father qualified as a solicitor and practised law in his grandfather George's office. However, he disliked the profession and, after his marriage and the death of his father, Walter passed over the practice to his brother Norman and took the lease of a hotel in the Isle of Wight. Unfortunately, the hotel did not prosper and was given up after a few years, and the family, which included Leslie's three sisters, Ivy, June and Molly, moved to Portsmouth. Although Leslie's father had a small allowance from his brother Norman and the legal practice in York, and he had various small intermittent incomes from teaching, the family was desperately poor during the 1930s. Leslie received little encouragement from his parents, but he passed the 11+ examination and entered Portsmouth Northern Grammar School in 1934, where he was awarded a very respectable School Certificate when he was 16 years old. However, it was now 1939 and World War II was about to start, and his school was evacuated to Winchester. With poor living conditions and little facilities for study, the young Leslie was determined to take a job and study part-time. He was appointed in 1940 as an assistant in the analytical laboratory of Timothy Whites and Taylor at their head office in Portsmouth under the supervision of Ron Gillham, who greatly influenced his further career; he was paid 13 shillings and 6 pence (in decimal terms, 67½pence) per week. In the evenings, Leslie studied at Portsmouth Municipal College for a London University Intermediate BSc. Alas, after a heavy bombing raid in January 1941, Timothy Whites and Taylor's laboratories were removed from the map, along with a great deal of the centre of Portsmouth—but fortunately not the MunicipalCollege.