William Mainwaring (6 Oct 1735 – 28 February 1821) was the MP for Middlesex from 1784 and chairman of the Middlesex and Westminster Quarter Sessions for a similar period. He was the eldest son of Boulton Mainwaring of Isleworth, Middlesex and educated at Merchant Taylors' School (1744–52). He then entered Lincoln's Inn in 1754 to study law and was called to the bar in 1759. He died in 1821, leaving one son.