The barony of Kilmarnock remained a subsidiary title of the earldom of Erroll until the death in 1941 of the eighteenth Earl's great-great-grandson, the twenty-second Earl.

The earldom, which could be passed on through female lines, was inherited by the late Earl's daughter and only child, the twenty-third Countess.

The barony of Kilmarnock, which could only be passed on to male heirs, was inherited by the Earl's younger brother, the sixth Baron.

He assumed the surname of Boyd in lieu of Hay the same year he succeeded to the title.

As of 2013[update] the title is held by his younger son, the eighth Baron, who succeeded his elder brother in 2009.