After a career in marketing with the Legal and General Assurance Society and involvement in local politics he was elected as Member of Parliament for Hove and Portslade 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.
Caplin is married with three children, Daniel, Jodie and Joshua.