Sadler was educated at Emmanuel College, Cambridge.
He was Master of Magdalene College, Cambridge from 1650 to 1660. He was nominated for Cambridgeshire for the 1653 Barebone"s Parliament. In 1659, for the Third Protectorate Parliament, he was Member of Parliament for Yarmouth, in the Isle of Wight.
Ernestine van der Wall writes:
The Hale Commission on law reform, headed from 1652 by Sir Matthew Hale, had Sadler as a leading lawyer, together with William Steele and John Fountain.
He was Town Clerk of London from 3 July 1649 (elected) to 18 September 1660. He was removed on the Restoration, under the pretext that he had signed the death warrant of Christopher Love.
He was suspended 4 September 1660, then the suspension was removed on 6 September 1660 and finally he was "declared incapable of office" on 18 September 1660. He wrote The Rights of the Kingdom (1649), a founding document of British Israelitism.
Tudor Parfitt calls it "one of the first invented expressions of an invented Israelite genealogy for the British".
This was not, however, its overt purpose. Glen Burgess calls it "an historical defence of the regicide". Maurice Vile writes
This interest was not clearly separated from the line taken by Sadler in The Rights of the Kingdom.