The son of Sir Henry Maynard, 3rd Bt., in 1751 Sir William married Charlotte [d. 1762], daughter of Sir Cecil Bisshop. They had two sons--Charles, who succeeded his father, and Henry, who became rector of Radwinter and vicar of Thaxted, in Essex. Around 1755-60, Maynard commissioned Arthur Devis to paint members of the Maynard family on the grounds of their estate, The Waltons (the painting is now in the NGA). He served from 1759-72 as a Member of Parliament for Essex. In 1766 he was granted the remainder of the dignities of Viscount and Baron Maynard, in default of male issue of his kinsman, Charles, 6th Baron Maynard (who was created on 18 Oct. 1766, Baron Maynard, of Much Easton, and Viscount Maynard, of Easton Lodge, both in the county of Essex).
Burke, Sir Bernard. A Genealogical and Heraldic History of the Peerage and Baronetage of the British Empire. 26th ed. London, 1864: 751.