[3] Matriculating at Trinity College, Oxford, on 6 November 1618, he left the university without a degree, and entered in 1620 Gray's Inn, where he was called to the bar in 1628, elected an ancient in 1645, and a bencher in 1649.
Newdigate was counsel with William Prynne and John Bradshaw on behalf of the state in the proceedings taken against Connor Maguire, 2nd Baron of Enniskillen, and other Irish rebels in 1644–5.
On 9 February 1653–54, he was called to the degree of serjeant-at-law, and on 31 May following was made a justice of the upper bench, in which capacity he was placed on the special commission for the trial of the Yorkshire insurgents on 5 April 1655.
He declined to serve, on the ground that levying war against the Lord Protector Oliver Cromwell was not within the statute of treason; and in consequence was removed from his place (3 May), and resumed practice at the bar.
He had succeeded in 1642, on the death of his elder brother, to that of Harefield, Middlesex, the ancient seat of his family, which had been alienated in the preceding century by his grandfather, a debtor, in a deal for Arbury with Edmund Anderson.