William was granted large tracts of land in Lancashire, in what was then called inter Mersam et Ripam, that is, "between the Mersey and the Ribble",[3] and the manors of Septon (Sefton), Thornton, Cuerden, ten carucates and a half of land, at the service of half a knight's fee.

[7] The Royalist gentry family held a large moated manor, a corn mill on the River Alt, and the advowson of St. Helen's Church at Sefton without interruption from about 1100 to 1700.

The office was held by Sir Robert's successors (descendants of his brother Richard), the Earls of Sefton until 1972.

It was the Lancashire line of the family that became the Viscounts Molyneux and later the Earls of Sefton, while there were also branches seated at Nottingham and Calais.

Control of the Castle finally passed out of Molyneux hands after Caryll had again been suspected of participation in a Jacobite plot.

Variations of the surname include "Mullinax", "Mullenax", "Molinex", "Mullinix", "Mullenneix", "Mullennix", "Mullineaux", "Molinieux", "Molinaux", "Molineaux", "Mollineaux", "Molineux", "Mulleneux" among others.