 He attended the common schools and Goshen and Newburgh (New York) Academies.He engaged in lumbering on the Delaware River and in mercantile pursuits and banking at Port Jervis, New York.He served as internal revenue collector and later as president of the Barrett Bridge Co..

St. John was elected as a Republican to the Forty-second and Forty-third Congresses (March 4, 1871 – March 3, 1875), after which he resumed his former business activities.