Father Costello was ordained to the priesthood on June 24, 1989.

Father Costello has served as a priest for more than 34 years, assigned as a parochial vicar at both Our Lady of Miracles and St. Athanasius, and as pastor at Immaculate Heart of Mary, Holy Trinity, St. Kevin (Administrator), and presently as pastor of St. Luke Roman Catholic Church.

Father Costello served four years as the Director of Family Life for the Diocese of Brooklyn and then as the Director of Jubilee 2000. He has also served on the Diocesan Evangelization Commission, as Associate Vicar for Clergy, and as a mentor for the newly ordained.

Father Costello served at the Pontifical North American College in Rome, and later as the Vice Rector of the former Cathedral Seminary House of Formation in Douglaston.

Father Costello is a former Spiritual Advisor to the Pro Sanctity Movement in Brooklyn, and a past member of the Holy Cross High School Board of Directors.