He was a Liberal specialist on foreign policy between the wars, and was one of the first to take a strong stand against Appeasement of the fascist dictators, and was a crusader on behalf of the League of Nations.

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).