In 2020, Mark was appointed chairman of CIB Limited, a subsidiary of Capital International Group. In December 1999 Field was selected to contest the safe Conservative seat of the Cities of London and Westminster following the retirement of former Northern Ireland Secretary Peter Brooke at the 2001 general election. Field won the seat with a majority of 4,499 and has been returned to Parliament with an increased majority three times since (2005 – 8,095; 2010 – 11,076; 2015 – 9,671). He was re-elected with a reduced majority at the 2017 general election. Field made his maiden speech in the House of Commons on 27 June 2001, when he declared his great political hero to be former Prime Minister Bonar Law. He was described by "The Guardian" as one of the most "hardline right wingers" up for election in 2001 after comments he made in 1991 about charities fighting the AIDS epidemic were reported. Field criticised AIDS campaigns as a waste of taxpayers' money and wanted mandatory tests for AIDS: "Many charitable trusts set up to help counter Aids in the mid-1980s became little more than a gay rights front", he wrote in "Crossbow" in 1991. As a parliamentarian Field, however, proved a strong supporter of equal rights. Within months of his election, in October 2001, he was one of four Tory MPs supporting a 10-minute rule bill on civil partnerships, a course he continued to support until it was on the statue books. He was also one of the Conservative MPs to vote in favour of gay marriage when this became law in May 2013. He was appointed an Opposition Whip by Iain Duncan Smith in 2003, becoming the Shadow Minister for London later that year. Between May and December 2005, he was Shadow Financial Secretary to HM Treasury. For 11 months, from late 2005 to late 2006, he was the Conservative Party's spokesman on Culture, Media and Sport under the new leadership of David Cameron in 2005. During his tenure he guided Opposition policy on the National Lottery Act 2006 and promoted policy safeguarding lottery funds for its four original causes of the Arts, heritage, charities and sport. He also led opposition to Britain's public library service. In September 2010, Field was appointed by the Prime Minister to the Intelligence and Security Committee, chaired by former Foreign Secretary, Sir Malcolm Rifkind. He became the youngest MP on this committee, which reports directly to 10 Downing Street and oversees the UK's intelligence and security services. He takes a special interest in economic affairs, financial services, foreign trade and international development and is chairman of the All-Party Parliamentary Group on Venture Capital & Private Equity as well as vice-chairman of the Groups on Football and Bangladesh.