When in November 1627 the five gentlemen who had been thrown into prison for refusing to contribute to the forced loan applied to the court of king's bench for a writ of habeas corpus, Calthorpe was counsel for Sir Thomas Darnell, being associated in the case with Noy, Serjeant Bramston, and Selden; and we are told that "the gentlemen's counsel pleaded at Westminster with wonderful applause, even of shouting and clapping of hands, which is unusual in that place".
In the conduct of this case he seems to have shown some lack of zeal, though when his turn came to speak he defended his client with conspicuous ability, notwithstanding that his sympathies were with the court party.
In December 1635 he succeeded Mason as recorder of London, the corporation having been specially requested to elect him in a letter which Charles I addressed to them on his behalf.
Shortly after this he was knighted, and was chosen to be reader of his inn, but he never discharged the duties of his office, causa mortalitatis, as Dugdale notes.
He was now in his fifty-first year, and his path seemed clear to the highest legal preferments, but death came upon him in the full vigour of his powers in August 1637.