Garnett was raised in the Anglican faith but became converted to Roman Catholicism and in 1575 went to Rome, where he joined the Society of Jesus and became a professor of Hebrew at the Roman College. He returned to England as a missionary in 1586, becoming Jesuit superior in England the following year. The extent of his role in the Gunpowder Plot is disputed. He was not an active conspirator but associated with the plotters on a number of occasions. When arrested and tried in 1606, he at first denied all knowledge of the plot but later admitted having learned of it under the seal of confession. He protested his innocence but was executed.