McTiernan was elected to the House of Representatives in 1929, but served for little over a year before Prime Minister James Scullin nominated him to the High Court.

[1] McTiernan left school in 1908 initially lacking the funds to attend university, instead joining the Commonwealth Public Service as a clerk.

He was rejected for military service during World War I due to an arm fracture sustained in childhood that had never properly healed.

This was to be short-lived, as in 1930, one year into McTiernan's term, Prime Minister James Scullin nominated him to the High Court of Australia, along with H. V. Evatt.

[1] As a judge of the High Court, McTiernan was involved in several significant cases in Australian legal history, including Bank of New South Wales v Commonwealth, which struck down an attempt to nationalise the banks, Australian Communist Party v The Commonwealth, which struck down an attempt to ban the Communist Party of Australia and R v Kirby; ex parte Boilermakers' Society of Australia, which reinforced the doctrine of the separation of powers.

He was the sole dissenter in Attorney-General (Vic) ex rel Dale v Commonwealth (1945), which struck down the Chifley government's Pharmaceutical Benefits Scheme and triggered a constitutional amendment.

[5] McTiernan had no intention of resigning from the bench even into the 1970s, but, after breaking a hip at the age of 84 in 1976 while chasing a cricket in his hotel with a rolled up newspaper, Chief Justice Barwick's refusal to include a wheelchair ramp in the design of the new High Court building prompted his retirement.

His inquiry is the third and most recent occasion on which a sitting High Court judge was called upon to investigate a matter on behalf of the federal government (usually regarded as a breach of separation of powers).

He did not marry until the age of 56, wedding Kathleen Lloyd on 27 December 1948 at St Roch's Catholic Church, Glen Iris, Melbourne.