The son of Joseph Greenwood Mabane and Margaret (née Steele) of Leeds, he was educated at Woodhouse Grove School and at Gonville and Caius College, Cambridge.

He entered the government as Assistant Postmaster-General under Neville Chamberlain in September 1939, an office he only held until October, when he was made Minister for Home Security.

When Winston Churchill succeeded Chamberlain as Prime Minister in May 1940, Mabane was appointed Parliamentary Secretary to the Home Department, a post he held jointly with Ellen Wilkinson from October of that year.

He later served under Churchill as Parliamentary Secretary to the Ministry of Food from 1942 to 1945 and as Minister of State for Foreign Affairs between May and July 1945.

He was sworn of the Privy Council in the 1944 New Year Honours[7] and a Knight Commander of the Order of the British Empire (KBE) in 1954.