He was a younger son of John Newdigate of Harefield Place, Middlesex, king's sergeant, and Amphelys, daughter and heiress of John Nevill of Sutton, Lincolnshire. He was educated at Cambridge, and on going to Court became and intimate friend of Henry VIII and a privy councillor. He married and had a daughter, named Amphelys, but his wife died in 1524, and subsequently he entered the London Charterhouse and became a monk there. He signed the Oath of Succession "in as far as the law of God permits", on June 6, 1534 but would go no further.
The government was at first anxious to secure the public acquiescence of the monks of the London Charterhouse in this matter, since for the austerity and sincerity of their mode of life they enjoyed great prestige. Having failed in this, the only alternative was to annihilate the resistance since a refusal engaged the prestige of the monks in the opposite sense. On May 4, 1535 the authorities sent to their death at Tyburn Tree three leading English Carthusians, John Houghton, prior of the London house, Robert Lawrence and Augustine Webster, respectively priors of Beauvale and Axholme. Little more than a month later, it was the turn of three leading monks of the London house: Humphrey Middlemore, William Exmew and Sebastian Newdigate. This process of attrition was to claim as its victims no less than fifteen of the London Carthusians.
Sebastian was arrested on May 25, 1535, for denying the king's supremacy, and locked up in the Marshalsea prison, where he was kept for fourteen days bound to a pillar, standing upright, with iron rings round his neck, hands, and feet. There he was visited by the king in person. Henry offered to load him with riches and honours if he would conform. He was then brought before the Council, and sent to the Tower of London, where Henry visited him again. Since he remained immoveable, his trial took place on June 11, and after condemnation he was sent back to the Tower. With his confreres Exmew and Middlemore he died at Tyburn Tree on June 19.
Sebastian Newdigate was beatified by Pope Leo XIII, on December 9, 1886.