In August 1587 he received orders, probably those of sub-deacon, and in September 1589, the year of his advancement to the priesthood, was sent back to Rheims to succeed William Giffard as professor of scholastic theology.
Afterwards he returned to Rheims, and having taken the degree of D.D., he was appointed in 1601 regius professor, and on 30 January 1606 magnificus rector or chancellor of the university.
When Arras College was founded at Paris in 1611 to associate scholars for the purpose of writing controversial works, Kellison was one of the five first admitted.
Kellison arrived at Douai on 10 June 1613, and in November he was installed as the fourth president of the college.
Within a few years he had expelled Edward Weston and Thomas Singleton, leading supporters of the Jesuits within Douai.