In 1856 he accepted the post of Private Secretary to the Right Hon. W. F. Cowper, afterwards Lord Mount Temple, and then President of the Board of Health in Lord Palmerston's Administration. In this office he served for two years, and then, retiring, he spent eleven months in foreign travel, visiting in turn the Tyrol, Venice, the Danube, Greece, Rome, Florence, and the Holy Land. During this period, he changed his plan of life, and in September, 1859, he was ordained Deacon by Bishop Lonsdale of Lichfield, on Letters Dimissory from Bishop Turton of Ely. His title was his Fellowship; but it was settled that the College should present him to the Vicarage of Great St. Mary's, Cambridge; and till it was vacant he was to have worked as a classical tutor in Trinity. Then came another change. "Dr. Vaughan's retirement," he wrote, "from the Head Mastership of Harrow startled us. We all took quietly for granted that he would stay on for years." However, this "startling" retirement took place, and there was a general agreement among friends of the School that Vaughan's favourite pupil, Montagu Butler, was the right man to succeed him. Accordingly, Butler was elected in November, 1859, though only twenty-six years old; and, with a view to the pastoral oversight of Harrow School, he was ordained priest, again by Bishop Lonsdale, at Advent, 1859. In January, 1860, Montagu Butler entered on his new duties at Harrow, and there he spent five-and-twenty years of happy, strenuous, and serviceable life. He found 469 boys in the School; under his rule the numbers increased till they reached 600. Butler's own culture was essentially classical, for he had been fashioned by Vaughan, who "thought in Greek," and he himself might almost have been said to think and feel in Latin elegiacs. But his scholarship was redeemed from pedantry by his wide reading, and by his genuine enthusiasm for all that is graceful in literature, modern as well as ancient. Under his rule the "grand, old, fortifying, classical curriculum," which Matthew Arnold satirized, fought hard and long for its monopoly; but gradually it had to yield. Butler's first concession was to relax the absurd rule which had made Latin versification obligatory on every boy in the School, whatever his gifts or tastes. At the same time he introduced the regular teaching of Natural Science, and in 1869 he created a "Modern Side." An even more important feature of his rule was the official encouragement given to the study of music, which, from an illicit indulgence practised in holes and corners, became, under the energetic management of Mr. John Farmer, a prime element in the life of the School.