The daughter of Sir Anthony Browne (June 27,1500-May 5,1548) and Alys Gage (d. March 31,1540), Mabel was a staunch catholic. Her marriage to the brother of her stepmother, Elizabeth Fitzgerald, on May 28, 1554 made her countess of Kildare. Gerald Fitzgerald (February 25,1525-November 16,1585) had been living in exile following the execution for treason of most of the other Fitzgerald men. He was restored to the title in 1554. During part of the reign of Mary Tudor, Mabel was a gentlewoman of the privy chamber. She was less welcome at court under Elizabeth. Living primarily in Ireland, Except for the occasions on which she accompanied her brother the Viscount Montague and his household.

Mabel had five children: Gerald, Lord Offalay (c.1559-1580), Henry, 12th earl of Kildare (d.1597), William, 13th earl of Kildare (d.1599), Mary, and Elizabeth.