Lord Dartmouth worked as a chartered accountant (FCA 1975), also the occupation of his father Gerald Legge, 9th Earl of Dartmouth, whose titles he inherited in 1997. As Earl of Dartmouth, he sat as Conservative peer in the House of Lords until 1999, when the Labour government of Tony Blair removed all but 92 hereditary peers from Parliament. In January 2007, Dartmouth announced he was leaving the Conservative Party in favour of the UK Independence Party (UKIP), citing concerns about the policies of David Cameron (then Leader of HM Opposition).

Dartmouth has a son, Gerald Glen Kavanagh-Legge (b. 2005), from his previous relationship with the BAFTA award-winning television Producer Claire Kavanagh. However, his son, being born out of wedlock, is not entitled to inherit the earldom, which passes only through legitimate male offspring, nor can he use the customary courtesy title of Viscount Lewisham.

In June 2009, Dartmouth married Melbourne-born former model Fiona Campbell, now styled Lady Dartmouth, whose first husband, Matt Handbury, is a nephew of Rupert Murdoch.

Dartmouth sits in the European Parliament as MEP for South West England, representing the UK Independence Party (UKIP) and served as one of two national Deputy Chairmen from 2016 to 2018.

Dartmouth was elected as the second UKIP MEP for the South West England region in the European Parliament election of 2009 and re-elected in 2014, as the first UKIP MEP on the regional list. In the European Parliament he sits with the Europe of Freedom and Direct Democracy group and on the Committee on International Trade. He has been the UKIP National Spokesman on Trade since 2010 and in February 2016 became one of the party's two national Deputy Chairmen. He is an author writing many publications for UKIP and the EFDD. He stood down as trade and industry spokesman on 22 January 2018 following UKIP's National Executive Committee vote of no confidence in leader Henry Bolton on the previous day.