Active in the Labour Party, Seddon worked for Gordon Brown during the 1992 General Election. After Brown became Chancellor of the Exchequer, Seddon served for five years on the Chancellor's Economic Policy Commission. Seddon was elected to Labour's National Executive Committee as a Grassroots Alliance candidate in 1997, gaining the highest share of the vote. Re-elected several times, he remained an NEC member until standing down in 2005. In the 2001 General Election, Seddon ran for parliament in the safe Conservative seat of Buckingham, against future Speaker John Bercow.