Mortimer is an English surname.
The origin of the name is Norman.
One version is that it derives from "Mortemer", site of the Cistercian Abbaye de Mortemer at Lisors near Lyons-la-Forêt and close to Rouen in Normandy. The land was granted to the Cistercians by Henry II of England in the 1180s. Finding the land to be a marshland area of the Lyons Forest around the running Fouillebroc stream, the monks dug out a large drainage lake and built the Abbaye de Mortemer. The ruins and lake can still be visited, and the later 16th century abbey hosts tours. According to François de Beaurepaire,
"There are two possible explanations: first, a small pond must have already existed before the land was given to the monks and have already been called Mortemer like the two other Mortemers, because the word mer 'pond' was not used anymore beyond the Xth century. This word is only attested in North-Western France and of Frankish or Saxon origin mari / meri 'mere', 'lake' (in Cambremer, Blingemer, etc..); mort(e) 'dead' is also quite common to mean 'stagnant' (in Port-Mort 'the port with stagnant water', Morteau 'dead water', etc.). Second, the monks could have given the name Mortemer to their drainage lake to remember the other Mortemer for any kind of reason we don't know, making a pun at the same time with Mer Morte 'Dead Sea'."
The village of Mortemer further north in the Seine-Maritime area bears the same name and predates the abbey at Lisors by more than one hundred years.