Sir William Middlebrook, 1st Baronet (22 February 1851 – 30 June 1936) was an English solicitor and Liberal Party politician.
William Middlebrook was born at Birstall in the West Riding of Yorkshire the son of John Middlebrook and Eliza Priestley. His mother was a distant relation of Joseph Priestley the philosopher, theologian and scientist. He was educated at Huddersfield College. In 1880 he married Alma Jackson from Morley, the daughter of William Jackson, the founder of the Peel Mills in Leeds. They had one son and two daughters.
Middlebrook went in for the law. He served his articles at Barton-upon-Humber and was admitted as a solicitor in 1872 or 1873. He began to practice in Birstall but later moved to Leeds and Morley, where he lived at Thornfield House, now the Masonic Lodge. He built up a large practice in which he was later joined by his son Harold.
Middlebrook held Liberal political views, perhaps strengthened by his active and lifelong membership of the Methodist Church. In 1883 he was elected a lay member of the Wesleyan Conference and in 1893 he became Treasurer of the Methodist General Chapel Committee. He entered Liberal politics and served as Hon. Secretary to the Spen Valley Liberal Association from 1885-95. In 1892 he was elected a member of Morley Town Council and was appointed an Alderman in 1894. He was Mayor of Morley in 1896 and in 1904 and he served as Mayor of Leeds in 1910-11. He was made an Honorary Freeman of Morley in 1919. While he was Mayor of Leeds, Middlebrook inaugurated a scheme for extending Leeds Infirmary under which nearly £130,000 was raised. He received the honorary freedom of Leeds in 1926.