Leonard Cottrell was born 2lst of May 1913 at Tettenhall, Wolverhampton to William and Beatrice Cottrell née Tootell. His father inspired an interest in history at the age of ten. At King Edward's Grammar School, Birmingham, Leonard was only interested in History and English, reading widely.
In the 1930s, Cottrell toured the English countryside on his motorcycle, visiting prehistoric stone circles, burial mounds of the Bronze Age, medieval and Renaissance monuments. On these journeys he was often accompanied by Doris Swain, whom he later married although the marriage was dissolved in 1962. After gaining experience writing articles on historical subjects for motoring magazines, he wrote his first documentary for the BBC in 1937.
Leonard was rejected by the RAF during World War II, for medical reasons, but he joined the BBC in 1942 and they stationed him, in 1944, in the Mediterranean with the RAF as a war correspondent. His experiences as war correspondent formed the basis of his book All Men are Neighbours (1947). He worked at the BBC until 1960, when he resigned and moved to a house overlooking the estuary of the River Kent in Westmoreland, Cumbria, where he stayed for the rest of his life, writing.
He was the editor of the Concise Encyclopaedia of Archaeology (1965).
Married and divorced twice, Doris Swain (in the 1930s, divorced 1962) and Diana Bonakis (married, 1965 - divorced, 1968). He had had no children by either marriage.