That same year he was one of the royal commissioners in Cornwall, where in August 1644 he entertained Charles I in his house.
During 1645 and 1646 he was in correspondence with Sir Edward Hyde, afterwards Earl of Clarendon, on the state of the royalist cause in Cornwall.
His son William Jane was Regius Professor of Divinity at Oxford.
Jane undertook to answer John Milton's ‘Eἰκονοκλάστης’ in a work ‘Eἰκών Ἂκλαστος; the Image Unbroken, a Perspective of the Impudence, Falsehoode, Vanitie, and Prophaneness published in a libel entitled "Eἰκονοκλάστης against Eἰκών Βασιλικὴ,"’ published in 1651 (without place).
Writing to Secretary Nicholas in June 1652, Edward Hyde said ‘the king has a singular good esteem both of Joseph Jane and of his book.’ Hyde shared this high opinion of the man, but doubted whether the book was worth translating into French, the better to counteract the effect of Milton's, as had been proposed.