William Mainwaring of Over-Peover was born circa 1286 in England to Roger Mainwaring (c1263-) and Christian Birtles (c1265-) . He married Mary Davenport (c1286-) .
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 from Original Evidences in Public Offices, the Harleian and Cottonian Mss., Parochial Registers, Private Muniments, Unpublished Ms.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-Peover, son and heir of Roger, married Mary, daughter of Henry Davenport, living 19 Edw. II., and had issue William Mainwaring, son and heir; Roger, Margery and Millicent, all living 8 Edw. III. 1334. This William died about 12 or 13 Edw. III."
Children
| Name | Birth | Death | Joined with |
| William Mainwaring (1316-1364) | 1316 England | 1364 England | Joan Praers (c1320-) Elizabeth Leycester (c1318-) |
| Roger Mainwaring | |||
| Margery Mainwaring | |||
| Millicent Mainwaring | |||
Siblings
| Name | Birth | Death | Joined with |
| William Mainwaring (c1286-) | 1286 England | Mary Davenport (c1286-) | |
| Joan de Mainwaring (c1290-) | |||