Troughton was born on 25 March 1920 in Mill Hill, Middlesex, England, to Alec George Troughton (1887–1953), a solicitor, and Dorothy Evelyn Offord (1886–1979), who married in 1914 in Edmonton. Patrick had an elder brother, Alec Robert (1915–1994), and a younger sister, Mary Edith (1923–2005). Troughton attended Mill Hill School and continued to live in Mill Hill for most of his life. While at Mill Hill School, he acted in a production of J.B. Priestley's "Bees on the Boat Deck" in March 1937. His brother A.R. ('Robin') Troughton shared the 1933 Walter Knox Prize for Chemistry with the future Nobel Prize winner Francis Crick, who also attended Mill Hill School.