Mary Grey, Countess of Kent (died 1 November 1702), suo jure 1st Baroness Lucas of Crudwell, was an English peeress in her own right.
Mary was the only surviving child of John Lucas, 1st Baron Lucas of Shenfield (1606–1671). On 2 March 1662/63 she married Anthony Grey, 11th Earl of Kent.
Following her marriage Mary was created, at her father's request, Baroness Lucas of Crudwell on 7 May 1663. This new creation was granted a special remainder to Mary's heirs male by Anthony Earl of Kent and failing which to her heirs female without division. This was a unique remainder for the English peerage. Unlike peerages created by writ this peerage can not fall into abeyance between female co-heirs but is inherited by the senior co-heir.
Mary, Countess of Kent and Baroness Lucas, died in November 1700 and was succeeded in the barony by her eldest son Henry, who in August 1702 succeeded his father in the earldom and who was created Duke of Kent 28 April 1710.