In 1856, he removed to New York City and practiced law there in partnership with his Yale classmate Sherman W. Knevals.

From 1866 to 1870, Phelps was an Assistant United States Attorney for the Southern District of New York.

In November 1872, Phelps was elected on the Republican ticket New York County District Attorney, and remained in office until his death, being re-elected in 1875 and 1878.

[4] In 1875, Carlotta Frances Shotwell testified before the Assembly Committee on Crime in New York City about the legal abuses in her 1874 trial.

Julius Catlin, and they had three children, including:[6] In October 1880, Phelps became ill, and never fully recovered.