He became the Assistant Secretary at the Department of Trade and Industry in 1981, before he was posted as the Commercial Counsellor at the British Embassy in Beijing, China.
He joined the Labour Party in 1996 and within nine months he was a member of Parliament, when he won the Wirral South by-election on 27 February 1997 on a 17% swing with a majority of 7,888.
Chapman ceased to be Caborn's PPS after the 2005 General Election and sat on the Intelligence and Security (Cabinet Office) Committee during the 2005–10 parliament.
On 17 May 2009, as part of a series of investigations by The Daily Telegraph into the expenses claimed by British MPs, Chapman admitted to an arrangement with Parliamentary authorities that had allowed him to claim interest payments on the entire amount of the mortgage on his designated second home in Lambeth, south-east London, despite having repaid £295,000 of the loan in 2002 and additional amounts thereafter.
[2] According to the Telegraph's investigation "between December 2002 and October 2003, Mr Chapman deliberately over-claimed for interest on the mortgage of his London house by about £15,000" and that "[astonishingly this was done] with the permission of an official in the Commons fees office".