Background
Mason was born in Falkirk, where his father Reverend James Aitchison was the senior minister of the Erskine United Free Church.
Mason was born in Falkirk, where his father Reverend James Aitchison was the senior minister of the Erskine United Free Church.
He graduated from Edinburgh with an Master of Arts in 1903 and an Bachelor of Laws in 1907.
Aitchinson became an advocate in 1907. He was particularly effective as a defence counsel in criminal cases, and was regarded as the best advocate before a jury since Sheriff Comrie Thomson. He was noted for the Bickerstaff and John Donald Merritt cases.
He was made a King"s Counsel in 1923.
He worked with Sir Arthur Conan Doyle and others to secure the release of Oscar Slater, the victim one of the most notorious miscarriages of justice of the early twentieth century. Aitchison who was leading Counsel at the appeal in 1929 gave a 14-hour speech.
Politics and law officer
An unsuccessful Liberal candidate for Clackmannan and East Stirlingshire in November 1922 and December 1923, he joined the Labour Party and contested The Hartlepools at the October 1924 general election and Glasgow Central in May 1929 — where he reduced a Unionist majority of nearly 6,000 to only 627. He was appointed as Lord Advocate in June 1929 serving in the Second Labour Government alongside Sir William Jowitt, the new Attorney General for England and Wales whose defeat at The Hartlepools in 1924 was attributed to Aitchnison"s drawing votes to the Liberals.
He was made a Privy Counsellor in 1929, and served as Lord Advocate until October 1933.
He was then raised to the bench as Lord Justice Clerk, with the judicial title Lord Aitchison, at which point he automatically resigned his seat in the House of Commons, which resulted in a by-election.
35th United Kingdom Parliament. 36th United Kingdom Parliament]
He was elected as Member of Parliament (Member of Parliament) for Kilmarnock at a by-election in October 1929, and sat for the constituency until October 1933 as a Labour then National Labour member.