Frederick Mallalieu's father, Henry (1831–1902), was a self-made businessman, at the age of twelve a hand-loom weaver, but becoming a woolen manufacturer, chairman of ironworks companies, and the magistrate.

[11] Mallalieu was a member of the main Liberal group in parliament led by Sir Herbert Samuel.

[12] After joining the Labour Party, he returned to the House of Commons in 1948, at a by-election on 24 March in the Brigg constituency, where he served as MP until he retired in 1974.

Sir William Mallalieu's daughter, Ann, has been a Labour life peer since 1991.

His uncle, Albert Henry Mallalieu, was head of that family of Tan-y-Marian, Llandudno.