He served in the Parliamentarian army during the First English Civil War, before resigning his commission when elected MP for Morpeth in 1645.

Excluded from Parliament by Pride's Purge in December 1648, he largely avoided politics thereafter but was appointed to Cromwell's Upper House in 1658.

[1] At some point after 1670, he married Susannah Hobbs (1657–1715); their fifth and only surviving son, Lawrence (1690–1742), succeeded his cousin as Fifth Viscount Saye and Sele in January 1710, by which date Fiennes had died.

[1] At the outbreak of the First English Civil War he joined the Parliamentarian side, and rose to the rank of the colonel of a regiment of horse by 1643, and served at the Battle of Naseby.

He was elected to the Long Parliament as member for Morpeth, probably in 1645, but was one of the MPs excluded in Pride's Purge in December 1648.