He was admitted as a solicitor in 1912 and worked for the firm of Burn and Berridge until his appointment to the Duchy of Cornwall.
[5] Burn served during World War I in the Sussex Yeomanry and the Machine Gun Corps (Cavalry) as 2nd Lieutenant.
At Gallipoli as a divisional machine gun officer, Burn was ordered to assist in the withdrawal at Gully Beach and was the last person in the regiment to leave the peninsula.
His father-in-law was Allen Stoneham who, with John Robinson Whitley, was instrumental in developing the golf and gambling resort of Le Touquet.
The couple had four children: Micky (1912–2010), Alan (1914–1998), Stella (1916–2004) and Renee (1919–1984), first wife of diplomat Lees Mayall.