In the First World War, Milligan served with the Highland Light Infantry from 1917 to 1919.

Milligan was a noted athlete and ran for the Achilles Club.

He was appointed Solicitor General for Scotland from 1951 to 1954,[3] and Lord Advocate from 1955[4] to 1960, and was made a Privy Counsellor in 1955.

He was appointed to the College of Justice in 1960, with the judicial title Lord Milligan.

Milligan was an unsuccessful parliamentary candidate at Glasgow St Rollox in 1945[5] and again at Central Ayrshire in 1950 and 1951, and was elected for Edinburgh North in a 1955 by-election,[6] where he served until 1960.