Caswall applied for promotion in the army or a government post but obtained neither. In the 1761 general election he was returned unopposed as Member of Parliament for Hertford on the interest of Nathaniel Brassey, his uncle. He continued to seek a salaried government post, but having sold his commission in the army in 1762, he disqualified himself in the King’s opinion. He married Mary Constantia Rolt on 28 January 1762, daughter and heir of Thomas Rolt of Sacombe Park. Mary's brother Captain Thomas Rolt (1st Foot Guards) the only son of Thomas Rolt, died at the battle at the Battle of St. Cast.
