He had two sisters, Frances (1595–1662) and Anne (1597–1639), as well as a half-sister from his father's first marriage, Elizabeth (born before 1593).

[1] Thomas Cromwell's father Edward escaped punishment for his role in Essex's Rebellion of 1601 but debt forced him to exchange his estates in England for lands in Ulster which had been confiscated after the end of Tyrone's Rebellion and relocate his family to the Kingdom of Ireland.

[3] A supporter of Charles I of England during the Wars of the Three Kingdoms, he was Colonel of Lord Cromwell's Troop of Horse, raised as part of the Royal Irish Army in 1640.

[4] In return for his service, he was created 1st Earl of Ardglass, in the Peerage of Ireland, on 15 April 1645; he was subsequently fined £460 by the Committee for Compounding with Delinquents set up by Parliament.

He married Elizabeth Meverell (died 1651), daughter and heiress of Robert Meverell of Ilam, Staffordshire, and of Throwley Old Hall, Staffordshire (died 5 February 1627/1628) and Elizabeth Fleming, both buried at Blore, Staffordshire, the daughter of Sir Thomas Fleming, Lord Chief Justice of the King's Bench, and had three children:[2][5] He and his wife both died in 1653 and were buried at St Peter's Church, Tickencote, Rutland, and his last will, dated 26 March 1653, was probated in 1661.