William was born about 1095. William's father was Roger Mainwaring. His paternal grandfather was Ld Rannulfus. He was an only child. William is no longer alive.
General Notes
vol 3, pg 229, Ormerod's "History of Cheshire"
pg 1260, Burkes Landed Gentry
pg 323, Burke's "A Genealogical and Heraldic History of the Extinct and Dormant Baronetcies of England, Ireland and Scotland" pub 1841