Born at Stanmore, Middlesex, on 2 August 1921, the son of Charles Bernard Ainslie MC (1879–1937) and his wife Eileen Holloway (d. 1962), Ainslie was educated at Harrow and Trinity College, Oxford, where he matriculated in 1947. His arrival at Oxford was delayed by his service in the British Army during the Second World War, and he was commissioned into the Royal Berkshire Regiment as a second lieutenant on 9 July 1942.
Ainslie's father was one of the ten children of William George Ainslie MP JP (1832–1893), himself one of the five children of Montague Ainslie JP DL (1792–1884), of Bengal in India and of Grizedale, Lancashire. William George Ainslie had been member of parliament (Conservative) for North Lonsdale in Lancashire from 1885 to 1892.
Ainslie's main career was as an arable, dairy and sheep farmer at Home Farm, Mildenhall, Wiltshire. In politics he was a socialist when he left school, and commented in 1985 "I voted Labour in 1945 and got disillusioned with the party by the end of the 1940s". In the 1950s he became a radical Liberal of the age inspired by Jo Grimond, and he was passionate about European unity, state education, and world development.
In 1962, with the support of others, Ainslie re-formed the moribund Devizes Constituency Liberal Association and established the Thrifty Orange, in Marlborough, an early charity shop, to raise funds for it.
In 1964, he was first elected to Wiltshire County Council, and he continued to represent his rural area at that level for nearly thirty years, until 1993. Although during those years the county was predominantly Conservative, while Ainslie was an outspoken Liberal, he served as chairman of the county's Education Committee from 1973 until 1977.
He stood unsuccessfully for Parliament in Devizes at the 1974 and 1979, and when in 1982 the SDP-Liberal Alliance agreed a division of UK constituencies between its two parties, it was a blow that he had to surrender Devizes to the SDP. Ainslie was a member of the local Alliance negotiating committee for Gloucestershire and Wiltshire, and he gave up his own position in Devizes to allow a younger Liberal to contest Stroud. However, he remained a strong supporter of the Alliance and later of the new Liberal Democrat Party.
In 1979, he contested the first direct elections to the European Parliament as a Liberal and in 1984 fought a strong SDP-Liberal Alliance campaign for the Wiltshire Euro-constituency, gaining over 60,000 votes. This campaign helped the Alliance to succeed at the county elections of 1985, when its members found themselves forming the largest group and Ainslie became Chairman of the County Council, a post he held until 1989. During the same period, he was simultaneously chairman of the county council's Policy (later Policy and Resources) Committee, which made him the de facto Leader of the council, as well.
After he died at Mildenhall on 5 January 2007, Sara Morrison, a former vice-chairman of the Conservative Party, said that it was "the end of an era... when we took our public duties and responsibilities very seriously", and the Labour peer Lord Faulkner of Worcester described him as "...a representative of the old school of parliamentary candidates in country areas; urbane, intelligent, knowledgeable on rural matters, a gentleman". Roger de Vere, Ainslie's successor as chairman of ARK, said "He was a great leader and a great man."
- Genealogy Data, dat7 page at ainslie.org.uk
- Oxford University Gazette, 26 April 2007, Obituaries at ox.ac.uk
- Graham, Christopher, Jack Ainslie, Old-school local government leader from The Independent dated Monday, 9 April 2007 (online edition)
- London Gazette, Gazette Issue 35698 (Supplement), 8 September 1942, Page 3956 at gazettes-online.co.uk
- Montague Ainslie page at sirjohnbarrowmonument.co.uk
- Genealogy Data, dat5 page at ainslie.org.uk
- Ncommons3 page at leighrayment.com
- Hugh Clayton, Emerging figure in new Lib-Lab pact An Alliance shire leader cast in Tory mould, in The Times, 11 May 1985; pg. 2; col C
- Former councillor dies from The Wiltshire Gazette and Herald, first published Tuesday 9 January 2007
- Life can be rough for a dog without the right to roam at telegraph.co.uk, 5 July 2001
- Wiltshire County Council, Clerk's Department page at nationalarchives.gov.uk
- Whitaker's Almanack for 1989, p. 280
- Charles Kennedy comes to Wiltshire dated Sunday, 24 June 2007, at duncanhames.org.uk