Judy Loe passed away on the night of 15 July in Beckinsale's arms after what was described as "immeasurable suffering".
Beckinsale shared the news in an emotive Instagram post, stating she was "paralysed" by grief and that her mother's death had been her "greatest fear".
Judy Loe had a distinguished career as an actor, notably as an original member of the musical Hair and appearing in numerous TV shows and films.
The announcement comes two years after Beckinsale revealed her mother had stage four cancer, though an official cause of death has not yet been disclosed.