As a young man, Dangerfield robbed his father, a farmer, and was imprisoned several times for counterfeiting and other crimes. Entering the pay of Mrs. Elizabeth Cellier, a Roman Catholic, Dangerfield took on the job of aiding Catholic suspects by defaming their accusers. But he soon betrayed his employer and publicly charged that, through Mrs. Cellier, Catholic nobles had paid him to assassinate Charles II and the Whig leader Anthony Ashley Cooper, 1st Earl of Shaftesbury. Although incriminating papers, planted by Dangerfield, were found in a meal tub at Mrs. Cellier's house, his testimony at her trial was so suspect that she was acquitted of treason.

Dangerfield continued to level charges against high-ranking Catholics, including Charles II's brother James, Duke of York. Finally, in 1685 he was convicted of libel for his anti-Catholic tract “Narrative” (1679). After being publicly pilloried and whipped, he was assaulted and struck in the eye with a cane by a barrister, Robert Frances; he died shortly afterward from the blow.