Acts 12:12 (ESV)

12 When he realized this, he went to the house of Mary, the mother of John whose other name was Mark, where many were gathered together and were praying.

Mary was the mother of John Mark. She was a wealthy woman since her house also has an outside gate (Acts 12:13; see Acts 10:17).  It is possible that her house was the same one used by Jesus and his disciples during the Last Supper (see Mark 14:51–52). The fact that no mention is made of her husband implies that she is a widow.