Derek Lang was born in Guildford, Surrey, England, on 7 October 1913, the son of Derek Lang. He was educated at Wellington College, Berkshire and the Royal Military College, Sandhurst. From there he was commissioned as a second lieutenant into the Queen's Own Cameron Highlanders on 31 August 1933. He served initially with the regiment's 1st Battalion before, promoted on 31 August 1936 to lieutenant, he served with the 2nd Battalion, Cameron Highlanders in Palestine during the Arab revolt. In February 1938 he was sent to Scotland where he became adjutant to the regiment's 4th Battalion, a Territorial Army unit serving as part of the 51st (Highland) Division, then commanded by Major General Victor Fortune.