Lambton was elected at the
1880 general election as a Liberal
Member of Parliament (MP) for
South Durham, 384 and held that seat until the constituency was abolished for the
1885 general election. He did not stand in
1885, but having joined the Liberal unionists in 1885 he unsuccessfully contested
Berwick-Upon-Tweed in
1886,
Sunderland in
1892, and a by-election in
South East Durham in February 1898. He was returned to the
House of Commons after a fifteen-year absence at the
1900 general election, when he defeated
Joseph Richardson, the Liberal winner of the 1898 by-election. Lambton was re-elected unopposed in
1906, but lost the seat by a wide margin to a Liberal candidate in
January 1910. Lambton inherited the earldom and a seat in the
House of Lords from his twin brother,
John Lambton, 3rd Earl of Durham, on 18 September 1928 when the latter died with no legitimate children. ==Marriage and children==