Dates of birth and death are not well-established. (Geni.com shows same birth year.) Her son Ranulph FitzRanulph became the lord of Spennithorne upon the death of Mary's husband. Her other son, Ralph FitzRanulph, then succeeded to the lordship of Middleham. That said, there remains some doubt as to whether the elder Ranulph (as linked below) was indeed the husband of this Mary. Her father, a Magna Carta Surety, is a 23X great-grandfather of Agnes Christian. In fact, most of those living with Agnes at the turn of the century (on lonely Pitcairn Island in the Pacific, the last refuge of the famous ship Bounty) were his descendants as well:
Sir Roger Bigod=Ida
Hugh Bigod=Maud Marshal
Isabel Bigod=Sir John FitzGeoffrey
Maud FitzJohn=William de Beauchamp

Isabella de Beauchamp=
Sir Patrick de Chaworth

Maud de Chaworth=Henry Plantagenet
Mary Plantagenet=Henry de Percy
Henry Percy=Margaret de Neville
Sir Henry "Hotspur" Percy=Elizabeth Mortimer
Elizabeth Percy=Sir John Clifford

Thomas Clifford=Joan Dacre
John Clifford=Margaret Bromflete
Henry Clifford=Florence Pudsey
Dorothy Clifford=Sir Hugh Lowther
Sir Richard Lowther=Frances Middleton
Sir Christopher Lowther=Eleanor Musgrave

Sir John Lowther=Eleanor Fleming
Agnes Lowther=Roger Kirkby
William Kirby=Joanna Furness
Eleanor Kirby=Humphrey Senhouse
Bridget Senhouse=John Christian
Charles Christian=Anne Dixon

Fletcher Christian=Mauatua
Thursday October Christian I=Teraura
Thursday October Christian II=Mary Young
Agnes Christian=Samuel Warren
Source: Weis, F.L., (line 164 in) Magna Charta Sureties (1999).∼Genealogical research for Mary (Bigod) FitzRobert, of Middleham and Spennithorn, is relatively small. She appears in Magna Carta Ancestry in a Richardson-documented trail from Gateway Ancestor Robert Drake to Magna Carta Surety Baron Roger le Bigod (vol. II, pages 90-92 DRAKE); which suggests she was the daughter of Roger Bigod, 2nd Earl of Norfolk and Ida (Toeni) le Bigod (Ida Plantagenet), born about 1188, in Thetford, Breckland Borough, Norfolk, England. In addition, it is suggested she was the sister of William (Plantagenet) Longespée [half-brother], William (Bigod) le Bigod, Margaret (Bigod) de Hastings, John Bigod, Hugh (Bigod) le Bigod, Roger Bigod and Ralph Bigod.

It is also suggested she married Ranulph (Taillebois) FitzRobert about 1199 (place unknown) and was the mother of Ranulf FitzRobert, Lord of Middleham and Ralph (Middleham) FitzRanulph (1210-1270). There is speculation that she died after 7 Dec 1252 at about age 72 in London, Middlesex, England, but her date of death and burial are unknown.

Provided by WikiTree and the Magna Carta Trail Project, which lists this as a pending project for Bigod and FitzRobert families, sources sited for the above information include:
Richardson, Douglas; Everingham, Kimball G., Editor. Royal Ancestry : A Study in Colonial and Medieval Families, Vols 1, pg 364. Salt Lake City, Utah. 2013.
Richardson, Douglas; Everingham, Kimball G., Editor. Royal Ancestry : A Study in Colonial and Medieval Families, Vols IV, pg 226. Salt Lake City, Utah. 2013.
Weis, F.L.. Magna Carta Sureties 1215, 2nd edn. (1955), page 127. Cites several other sources.