Born at his grandfather’s Suffolk property of Rishangles, near Thorndon,
Following the death of his brother-in-law Christopher Herrys in October 1628, Grimston was returned to Parliament for Harwich, presumably on his father’s interest, but played no recorded part in the 1629 session. Called to the bar in November 1628, he subsequently acquired the recorderships of both Harwich and Colchester. A parliamentarian of Presbyterian sympathies during the civil wars of the 1640s, Grimston represented Colchester during the Short and Long Parliaments, but was secluded at Pride’s Purge after urging the Commons to ratify the Treaty of Newport and Bradfield Hall was plundered. Forced into early retirement, he purchased a new seat at Gorhambury, in Hertfordshire and edited the law reports of his father-in-law, which were published in 1655. Elected to the second Protectorate Parliament for Essex in 1656, he was again barred from taking his seat, but was readmitted to the Long Parliament in February 1660. Later that year he was appointed to the council of state, by which time he may already have opened secret communications with the royalist Court in exile. As Speaker of the Convention he greeted Charles II on the latter’s arrival at Whitehall in May 1660. His part in the Restoration was rewarded with the mastership of the Rolls.
