Born in Eltham, to a formerly affluent family which was severely affected by the Wall Street Crash in 1929, and his father's long-term illness and early death ending Jackson's formal education. He joined the GPO Film Unit on his 17th birthday as a messenger boy after his mother persuaded her MP, Sir Kingsley Wood, then also postmaster general, to find work for her son. Rising to production assistant, he was part of the crew for the short film "Night Mail" (1936). The voice narrating the poem by W.H. Auden ("This is the Night Mail crossing the border, bringing the cheque and the postal order.") was Jackson himself. He directed a number of documentaries, the first being "The Horsey Mail" (1938) about the rural postal service in Suffolk. "The First Days" (1939), co-directed by Harry Watt and Humphrey Jennings, was the first of the wartime documentaries, in this instance concerned with the 'Phoney War' period.