He attended City College of New York and Hebrew Union Teachers College. He graduated from New York University School of Law in 1924, and practiced law in New York City.
During the First World War Farbstein served in the United States Coast Guard Reserve. On September 18, 1947, Farbstein married Blossom Langer, and they had one son: Louis Farbstein (born 1948). He was elected as a Democrat to the 85th, 86th, 87th, 88th, 89th, 90th and 91st United States Congresses, holding office from January 3, 1957, to January 3, 1971.
He was an open supporter of Lyndon B. Johnson"s policy on the Vietnam War.
In 1970, he was defeated for re-nomination by Bella Abzug. Farbstein died on November 9, 1993, in New York City, and was interred in Cedar Park Cemetery in Paramus, New Jersey.
He was a member of the New York State Assembly (New York Company, 4th Doctorate) in 1933, 1934, 1935, 1936, 1937, 1938, 1939-1940, 1941-1942, 1943-1944, 1945-1946, 1947-1948, 1949-1950, 1951-1952, 1953-1954 and 1955-1956.