Born on 16 February 1927, St Clair was the son of Major-General G.P. St Clair CB CBE DSO. He was educated at Eton College. After leaving school, St Clair joined the Royal Armoured Corps as a trooper and in 1946 was commissioned as an officer into the Royal Scots Greys. He left the Army in 1948.
Political career
St Clair served as honorary secretary to Winston Churchill from 1948 to 1950, before returning to run his family's dairy farm at Tetbury in Gloucestershire. In 1955, he stood unsuccessfully as a Conservative candidate at the London County Council elections, in Islington East. At the 1959 general election he stood as Conservative candidate in Bristol South East, but he lost to the sitting Labour Member of ParliamentTony Benn, whose majority was nearly 6,000 votes. However, in November 1960 Benn's father died and Benn inherited his peerage as Viscount Stansgate, with an automatic seat in the House of Lords. This disqualified Benn from sitting in the House of Commons, triggering a by-election on 4 May 1961. Benn, who wished to be allowed to disclaim his peerage, defied his inability to sit in the Commons by standing at the election, and he and St Clair were the only two candidates. St Clair's campaign displayed posters near every polling station warning voters that Benn was disqualified and that any votes for him would have no effect. Benn nevertheless won the election with nearly 70% of the votes and an increased majority of over 13,000. However, an Election Court considered what to do about the result, found that Benn was disqualified from being elected and that the voters were aware of this, and awarded the seat to St. Clair as the only duly qualified candidate. Outside Parliament, Benn continued to campaign for a change in the law to allow him to disclaim his peerage and return to the Commons, and eventually the Conservative government agreed. The Peerage Act 1963, making such a change in the law, was given the Royal Assent and became law shortly after 6 p.m. on 31 July 1963. Benn was the first peer to disclaim his title, at 6.22 p.m. that day. St Clair had already given an undertaking that he would respect the wishes of the people of Bristol if Benn became eligible to take his seat again, so he immediately resigned by accepting the office of Steward of the Manor of Northstead. This triggered another by-election at which St Clair did not stand, and indeed there was no Conservative or Liberal candidate. On 20 August 1963, Benn was returned to the Commons with nearly 80% of the votes.
In June 1955, St Clair married Rosalie Alice, daughter of Wing-Commander C.L. Hargreaves and granddaughter of Alice Liddell. He lived at Upton House, Tetbury, Gloucestershire, and 28 Chesham Place, London. He was a member of the Cavalry Club.
