Tresilian appears in the records for the first time in 1354. His early career took place in Oxfordshire and Berkshire; in 1367 he was a Justice of the Peace (JP) in Berkshire and in 1368 in Oxfordshire. He also worked in his home county, and in 1370 was a JP for Cornwall. In the 1370s he began working in royal administration, and in 1378 he was made Justice of the King's Bench. Shortly after he was also knighted. When Chief Justice Sir John Cavendish was killed in the Peasants' Revolt in 1381, Tresilian was appointed to take over the position.
After the rebellion was over, Tresilian was put in charge of punishing the rebels, and did so extremely harshly. He followed King Richard II into Essex, where he led what was described as a 'bloody assize' against the rebels. He pressured jurors into giving up names of suspects, and to maximise sentences, contrived to have charges presented as felonies rather than trespasses. All in all nineteen men were hanged, while another twelve were hanged and drawn. There was a widespread belief in the localities that royal retribution had gone too far, and that reform of government was necessary as well as punishing the rebels, to prevent further uprisings.
In the following years Tresilian became increasingly involved in politics, as a loyal follower of the king. In November 1386 parliament appointed a commission to review and control royal finances. The king resented this infringement of his royal prerogative, and in the so-called 'questions to the judges', he received legal backing for the position that the commission was unlawful. It is largely assumed that it was Tresilian who drafted the 'questions', and thereby turned a political controversy into a legal dispute. The king's opponents went on the counterattack, and on 17 November 1387, Tresilian was among a number of loyalists who were charged with treason by the group of noblemen known as the Lords Appellant. When Tresilian's case came up for trial, he had gone into hiding and was not to be found, and was sentenced in absentia. Not long after he was discovered in hiding in sanctuary in Westminster. He was dragged into court with cries of 'We have him!' from the mob, and as he was already convicted, he was summarily executed. He was hanged naked before his throat was cut.
The charges against Tresilian had consisted in more than simply treason. He was a highly unpopular judge, and among his crimes was also corruption. Several cases were presented from Cornwall and Devon, where the judge had abused his powers to advance his own fortune. Tresilian and his wife Emmaline (Emma), had a son John, and at least two daughters. Through his marriage, but also through corrupt conduct, he had acquired great parts of land in Berkshire, Buckinghamshire, Oxfordshire, and Cornwall. His land was forfeited at his death, but, to his son's objections, much of it was regained by John Hawley the elder, a merchant and pirate from Dartmouth, who married Emma.