Proskauer married Alice Naumburg on October 14, 1903, and they had three children: Frances, Ruth, and Richard.

After being admitted to the bar, Proskauer and a friend formed a law firm, which in 1902 merged with James, Schell & Elkus to create a firm that later became Elkus, Gleason & Proskauer.

Early in his career, Proskauer became a friend of New York Governor Al Smith and served as his campaign advisor and speechwriter.

In 1927, he was appointed an appellate judge and served in that position until he retired from the bench in 1930.

From 1951 to 1953, he chaired the New York State Crime Commission investigating organized crime in New York City, and he was involved in the creation of the Waterfront Commission.