Onderdonk was born in New York City. He studied at Columbia University, receiving his degree in 1805, and then traveled to Britain for further education, receiving his medical degree from the University of Edinburgh. On returning to the United States, Onderdonk practiced medicine in New York before being ordained to the deaconate and priesthood by Bishop John Henry Hobart. In 1816, he went to western New York as a missionary and then returned east to become rector of St. Ann's Church in Brooklyn, remaining there for seven years.
Bishop of Pennsylvania
Onderdonk was elected assistant Bishop of Pennsylvania in 1827, serving initially as assistant to Bishop William White. He was the 21st bishop of the ECUSA, and was consecrated by bishops William White, Alexander Viets Griswold, and James Kemp. However, bishop Kemp died of injuries received in a stage coach accident while returning from the consecration, so Onderdonk substituted in the Episcopal Diocese of Maryland until a successor was elected.
On Bishop White's death in 1836, Onderdonk became bishop. Onderdonk was a strong advocate of the pre-Tractarian High Church position, in company with his brother Benjamin Treadwell Onderdonk, who was also a bishop. In 1844, Onderdonk was suspended from the exercise of his episcopal office after rumors of alcoholism. The suspension was lifted in 1856, two years before his death.
- Batterson, 94
- Batterson, 95
- George Freeman Bragg, The First Negro Priest on Southern Soil (Baltimore: Church Advocate Press, 1909) p. 13, available at google books
- Batterson, Hermon Griswold (1878). A Sketch-book of the American Episcopate. Philadelphia: J. B. Lippencott & Co. Retrieved 2009-07-30.
- The Episcopate in America, by William Stevens Perry