He entered the household of Sir Francis Bryan in the 1530s after studying law at Furnival's Inn, but was dismissed in 1538 because of his advocacy of religious reform.
[4] Despite this advice to his co-religionists to let matters take their own course, Lassells was personally involved in the following year in the downfall of Norfolk's niece, Queen Catherine Howard.
Lassells immediately reported Mary's words to Archbishop Thomas Cranmer, setting in motion a process which ended with the Queen's execution.
According to Ryrie, Lassells "maintained that he revealed the information to avert a charge of misprision of treason, which may well be true, but he can hardly have regretted the destruction of so prominent a Howard".
He was denounced as a patron of Richard Laynam, "a London prophet who predicted the imminent overthrow of the King", is described by John Bale as the "instructour" of his friend, the sacramentarian Anne Askew, and is considered by a modern historian, A. G. Dickens, to have been the "leading spirit" of the radical group at court.