Denis Garrett was born in 1906 at Leiston, Suffolk, to Mary (née Marples), from a Sheffield tool-making family, and Stephen Garrett (1878–1915), a director in the family's agricultural machinery business in Leiston, Richard Garrett &amp; Sons. He was the eldest of four children. His father was killed in action at the Battle of Neuve Chapelle during the First World War when he was eight, and his mother moved the family to Oxford, where her sister lived, and later to Eastbourne. Garrett attended the Dragon School in Oxford, and briefly Wellington College, from which he ran away, and Eastbourne College, where he became interested in plant biology, despite a lack of biology classes, as well as mathematics.