He studied law, was admitted to the bar in 1886 and commenced practice in Nyack, New York.

Tompkins was elected chairman of the Rockland County Republican committee in 1888.

Tompkins was elected justice of the Supreme Court of New York in 1906.

He was promoted to the Appellate Division of the Supreme Court of New York in January 1930 and served until his retirement in 1936.

[1] This article incorporates public domain material from the Biographical Directory of the United States Congress