William Mainwaring was born circa 1210 in England to Roger de Mainwaring (c1185-) .
The History of the County Palatine and City of Chester: Comp. from Original Evidences in Public Offices, the Harleian and Cottonian Mss., Parochial Registers, Private Muniments, Unpublished Ms. Collections of Successive Cheshire Antiquaries, and a Personal Survey of Every Township in the County; Incorporated with a Republication of King's Vale Royal and Leycester's Cheshire Antiquities, Volume 1 by George Ormerod, Daniel King, William Smith, William Webb (sheriff.), Sir Peter Leycester, 1882
"William Mainwaring, of Over-Pever, younger son of Roger Mainwaring of Warmincham, had Over-Pever by the gift of his father regnante Henrico Tertio. He had issue, William, son and heir. Thomas to whom his father gave Fodon in Over-Pever; Benedict another son; Guy another son. Also Roger, another son whose son William married Margaret, daughter of Roger Toft of Toft."
Children
| Name | Birth | Death | Joined with |
| William Mainwaring (c1235-) | 1235 England | ||
| Thomas Mainwariing | |||
| Benedict Mainwaring | |||
| Guy Mainwaring | |||
| Roger Mainwaring | |||
Siblings
| Name | Birth | Death | Joined with |
| Thomas Mainwaring | |||
| Roger Mainwaring | |||
| William Mainwaring (c1210-) | 1210 England | ||