Pursuing his studies with the utmost zeal and ardour at the Jesuits' College, under the celebrated Bellarmine, and the no less celebrated Clavius, he made such progress, that upon the indisposition of the latter, he was able to fill the mathematical chair.

Nor was he less skilled in philosophy, metaphysics, and divinity; and his knowledge of Hebrew was so profound that he taught it publicly in the Roman schools.
To an enthusiastic zeal in the cause of the religion he had espoused, Garnet added great powers of persuasion and eloquence,--a combination of qualities well fitting him for the office of a missionary priest; and undismayed by the dangers he would have to encounter, and eager to propagate his doctrines, he solicited to be sent on this errand to his own country.

At the instance of Father Persons, he received an appointment to the mission in 1586, and he secretly landed in England in the same year.

Braving every danger, and shrinking from no labour, he sought on all hands to make proselytes to the ancient faith, and to sustain the wavering courage of its professors.

Two years afterwards, on the imprisonment of the Superior of the Jesuits, being raised to that important post, he was enabled to extend his sphere of action; and redoubling his exertions in consequence, he so well discharged his duties, that it was mainly owing to him that the Catholic party was kept together during the fierce persecutions of the latter end of Elizabeth's reign.
Compelled to personate various characters, as he travelled from place to place, Garnet had acquired a remarkable facility for disguise; and such was his address and courage, that he not unfrequently imposed upon the very officers sent in pursuit of him.