Quite separately, Humphreys has pursued a career as Writer of popular fiction. Starting with Sleeping Partner in 2000, he received praise from Colin Dexter, who stated that the book was: "[a] splendid debut in crime fiction - more please, Mr Humphreys".

Humphreys wrote Riptide in 2001, which the Observer said to be "an accomplished, intriguing story". In 2010, he published a book of ghost stories, called The Mincer and Other Stories in order to raise funds for the Woodland Trust.

Humphreys worked as a civil servant, negotiating environmental legislation with the European Union and later becoming Head of Corporate Communications at the Prime Minister's Office. He was Professor of Government at City University London from 2005 to 2009.

He is a member of the Green Party and became chair of the party in 2008. He stood for election in Islington South and Finsbury in the General elections of 2005 and 2010 and for Islington Council also in 2010. He is an adviser to the Green Member of Parliament Caroline Lucas.