[1] Marks was educated at Harrow School and University College, Oxford.

Following the creation of the Liberal Democrats, Marks served as a member of the party's Committee for England in 1988–89.

He was a member of the Federal Policy Committee from 2004 to 2015 and was Chair of the Liberal Democrat Lawyers Association from 2001 to 2007.

(Cohabitation Rights Bill [HL])[4] In 1982, Marks married Sarah Ann Russell, with whom he has one son and one daughter.

[5] Marks owned Holmwood, a country house in the Oxfordshire village of Binfield Heath, from 2008 to 2018.