He was then appointed to serve as a missionary in Annandale-on-Hudson, New York, where he remained till July 1861.

While there, Seymour saw the building of a church and a training institution for preparation for the ordained ministry as part of St Stephen's College.

Then, between 1863 and 1867, he served as rector of St John's Church in Brooklyn.

He was also instrumental in retaining the presence of the seminary in Manhattan rather than relocating in a rural location.

He was then consecrated at Trinity Church in New York City on June 11, 1878, by Bishop Horatio Potter.