[1] In January 1643, during the First English Civil War he was appointed Commander-in-Chief of the forces of the Parliament in the Midland Counties and Governor of Leicester.

[2][1] In 1648 he won some credit for his share in the pursuit and capture of the Duke of Hamilton; he assisted Colonel Pride in purging the Parliament by helping to identify members to be excluded.

[citation needed] A member of the Council of State under the Commonwealth, Lord Grey of Groby fought against the Scots in 1651 during the Third English Civil War.

In 1628 Thomas at the age of five acquired the courtesy title of Lord Grey of Groby when his father was created the 1st Earl of Stamford.

With the English Civil War looming, in 1641 Grey was elected a Member of Parliament for Leicester in the House of Commons and was admitted to Gray's Inn like his father before him.

[1] On 16 January 1643, Thomas, Lord Grey of Groby was appointed commander-in-chief of the midland counties association and then ordered to take special care of Nottingham during the second Civil War.

[1] By end of the year 1643, Grey's views began to diverge from his father's moderate ideas and in 1644 he left Leicester because of misunderstanding with the county.

[3] In 1645 Lord Grey was forced to give up command by the Self-Denying Ordinance of April 1645 and it was after this that he entered a political alliance with radical and republican politicians, also petitioning to meet the royalist attack.

On 6 December 1648 Lord Grey assisted with Pride's Purge (pointing out 'obnoxious' members who were to be removed from the house), he was also a supporter of the Leveller Cause.