The family of the seneschals of Rennes begins with the seneschal Mainfinit (c.1060–95), but the counts of Rennes/dukes of Brittany employed a household seneschal from the first half of the eleventh century, and the office seems have become hereditary by the middle of the century. This is the conclusion to be drawn from the fact that Mainfinit acquired his office by marrying Commater ‘senescalca’, the widow of the seneschal Geoffrey son of Glai, so that ‘redditus est ei omnis honor’ qui ad senescalciam Gauffredi pertinebat' (BN ms fr. 22331, p. 236).

Mainfinit in fact originated in Nantes and possibly joined the comital household when Conan II was briefiy acknowledged as count of Nantes from 1050 to 1054 (Preuves, cols. 409, 484; A. Chédeville and N.-Y. Tonnerre, La Bretagne féodale XIe-XIIIe siècle, Rennes, 1987, pp. 42–3). Mainfinit first appears with the title ‘siniscaldus Redonensis’ in the reign of Count Geoffrey Grennonat (1066–84) (Preuves, col. 428), presumably fol- lowing his marriage to the widow of Geoffrey son of Glai. Mainfinit had at least three sons: William, Walter (the name of Mainfinit's brother) and Agaat (Preuves, cols. 428, 463, 484, 566). Mainfinit disappears after going on the First Crusade in the entourage of Duke Alan IV (Preuves, col. 484). On the duke's return there appears a new seneschal, William (Preuves, cols. 504–5, 512).