In English law, the assize of mort d'ancestor ("death of ancestor") was an action brought where a plaintiff claimed the defendant had entered upon a freehold belonging to the plaintiff following the death of one of his relatives. The questions submitted to the jury were, 'was A seised in his demesne as of fee on the day whereon he died?' 'Is the plaintiff his next heir?' This assize enabled the heir to obtain possession, even though some other person might have a better right to the land than the deceased.
It was one of the so-called "petty assizes" established by the Assize of Clarendon by Henry II in 1166 along with the Assize of Northampton (1176). By the Assize of Northampton, the lord must not prevent the heir having seisin forthwith on the ancestor's death, making this almost the final step in the development of common law heritability. Like the other two assizes, it was abolished in 1833.
Two early instances of such an action are recorded in feet of fine from the reign of King John for a family dispute between members of the de Brantingham family in Yorkshire in 1202. On 22 August 1202, one Matilda (or Maud), daughter of John de Brantingham, brought an action under the assize of mort d'ancestor against her sisters, Mary and Alice de Brantingham. Less than four months later, on 1 December 1202, John de Brantingham, son of Haldane the Deacon (and not to be confused with the later John de Brantingham, a Yorkshire clergyman), brought a similar action against his three daughters.