 On February 1, 1909, Grimes was appointed Coadjutor Bishop of Syracuse and Titular Bishop of "Hermeria" by Pope Pius X.He received his episcopal consecration on the following May 16 from Archbishop John Murphy Farley, with Bishops Patrick Anthony Ludden and Thomas Burke serving as co-consecrators.Upon the death of Bishop Ludden, Grimes succeeded him as the second Bishop of Syracuse on August 6, 1912.