Mander was expected to be nominated Chief Whip for the Liberal Party in the House of Commons, but he lost his seat at the 1945 general election, in the post-war Labour landslide.

He was chairman of Mander Brothers (established in 1773) for a generation, one of the principal local employers and a major manufacturer of paints, inks and varnishes in the British Empire.

His second wife, Rosalie Glynn Grylls, was a biographer and lecturer with an interest in the writers and artists of the Romantic period, and an early connoisseur of the Pre-Raphaelite movement.

Her biographical subjects included Mary Shelley (1938), Claire Clairmont (1939), Edward John Trelawny (1950), William Godwin (1953), Dante Gabriel Rossetti (1964), Ivy Compton-Burnett (1971) and Elizabeth Barrett Browning (1980).

His autobiography was published posthumously in 2021 as Lemons for Chamberlain: The Life and Backbench Career of Geoffrey Mander MP, edited by Patricia Pegg.