Caplin was elected as Member of Parliament for Hove in 1997. In 1998 he became Parliamentary Private Secretary to Margaret Beckett. After re-election in 2001 he became an Assistant Government Whip and then Parliamentary Under Secretary of State and Minister for Veterans at the Ministry of Defence. Following the invasion of Iraq in 2003 Caplin was the focus of anti-war protests mostly coordinated by "Hove Action for Peace". Large weekly protests were held outside his Constituency Surgery at Hove Town Hall over a few years. As a Whip and Junior Defence Minister many people in Brighton and Hove felt that he was directly complicit in the decision to go towar in Iraq. He gained a certain notoriety and was heavily criticised following his refusal to accept an anti-war petition presented to him from the people of Hove. During his term as Parliamentary Under-Secretary of State for Defence, he introduced the Veteran's Badge for those who had served in HM Armed Forces. He stood down from the House of Commons at the 2005 general election. In 2010 Sir Thomas Legg conducted an independent audit of MPs expenses in which he determined that Caplin should repay £17,865 representing mortgage interest payments claimed on his second home, after Caplin failed to provide paperwork to establish his entitlement to claim the money. Caplin appealed, explaining that Legg's letters had gone to an old address and had not been forwarded. On 1 April 2011 his appeal was upheld and the amount of repayment reduced to £1,178.43 ; Caplin accepted the finding and agreed to repay the money.
Post parliamentary career
Caplin now runs his own company Ivor Caplin Consultancy Ltd. which has a number of clients including Foresight Communications, a political lobbying firm whose clients include EADS, EDS and Sodexho. He has recently founded a new company International Infrastructure Development Consultancy Ltd. which is specialising in advice and support for non EU Governments and the private sector overseas. With effect from August 2009 Caplin was appointed as Chief Executive of Mayfair Capital Management a private group of UK companies based in London with executive responsibility for the construction, energy and property divisions of the group. He was the Chair of the Jewish Labour Movement for 2018–19.