From 1923 to 1924, Phelps held the post of Deputy Commissioner for Veterans Affairs for New York City. He served as a Republican member of the New York State Assembly from Greenwich Village in 1924, 1925, 1926, 1927 and 1928. In 1928, he ran for a nomination for the United States House of Representatives, but lost the primary to Ruth Baker Pratt, who would become the first congresswoman from New York. Following the 1932 Republican National Convention, Phelps became a Democrat, and travelled to the 1936, 1956, and 1960 Democratic National Conventions as a delegate. He was again a member of the State Assembly (New York Co., 3rd D.) in 1937 and 1938; and a member of the New York State Senate (13th D.) from 1939 to 1942, sitting in the 162nd and 163rd New York State Legislatures.