Following his retirement from politics, Ainsworth was appointed UK Chair of the Big Lottery Fund and, later, Chairman of the Churches Conservation Trust.
The son of a naval officer, Ainsworth was educated at the Ludgrove School in Wokingham, Bradfield College, and Lincoln College, Oxford, from which he graduated in 1979, with an MA in English Literature and Language.
On leaving university he became a researcher to the former Conservative Member of the European Parliament, Sir John Stewart-Clark, and then in 1981 became a merchant banker. He worked as an investments analyst for Laing & Cruickshank Investment Management (bought by UBS in 2004) from 1981 to 1985, and then in corporate finance for S.G. Warburg Securities (bought by UBS in 1994) from 1985 to 1992, where he became a director from 1990 to 1992.
He married Claire Burnett in Hatfield in 1981, with whom he has a son, Benny Ainsworth (actor) and two daughters.
He was elected as a councillor to the London Borough of Wandsworth in 1986 and at the 1992 general election was elected to Parliament for the seat of East Surrey, succeeding Sir Geoffrey Howe.
In 1994 Ainsworth became the Parliamentary Private Secretary (PPS) to the Chief Secretary to the Treasury, Jonathan Aitken, and in 1995 became PPS to the Secretary of State for National Heritage, Virginia Bottomley. He was promoted by John Major in 1996 to the Whips' Office. When the Major government fell the following year he remained a whip in opposition and was promoted to Deputy Chief Whip by William Hague.
On 5 January 2010, Ainsworth announced that he was to stand down at the forthcoming general election. The Conservative majority in East Surrey was 15,921 in 2005.
In 1998 he entered the Shadow Cabinet, shadowing the Department for Culture, Media and Sport and from 2001 the Department for Environment, Food and Rural Affairs. Ainsworth resigned from Iain Duncan Smith's frontbench for family reasons in 2002.
From 2003 he chaired the Environmental Audit Select Committee, before rejoining the Shadow Cabinet under David Cameron in December 2005 as Shadow Secretary of State for Environment, Food and Rural Affairs. The post has had a heightened importance given the Conservative's new emphasis on environmental policies under Cameron.
Speaking in March 2006, Ainsworth set out the possible new direction for Conservative policy stating that "Achieving a sustainable world and combating the threat of climate change will require some really fresh ideas and radical thinking. We cannot expect to meet the challenges of this century by toying with the structures and technologies we have inherited from the past, and the concept of Decentralised Energy should to be taken seriously." Ainsworth was notable as the only member of the shadow cabinet to have voted against the war in Iraq.
Ainsworth lost his position in the Shadow Cabinet in the January 2009 reshuffle when Nick Herbert took the post of Shadow Secretary of State for Environment, Food and Rural Affairs.
In June 2011 Ainsworth was appointed chairman of the Big Lottery Fund and recently became Chairman of the Churches Conservation Trust. Ainsworth continues to be a board member of the Environment Agency and was previously Chairman of the Elgar Foundation and the wild plant charity Plantlife.