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. 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. Later in 1648, he was made commissioner of the court which tried King Charles I. His signature on the death warrant indicates that he was a strong advocate for the execution of the King, because he signed after the President of the court John Bradshaw and before Oliver Cromwell, who was third to sign out of a total of fifty nine commissioners (judges). Grey was the only person of nobility to sign the death warrant.
