While in the Assembly, Byrnes accused some of Governor
Herbert H. Lehman's anti-crime bills as "crackpot" legislation and a "hodge-podge of ill-advised measures." Lehman then singled him out in a message to the Legislature on better law enforcement, calling him a reactionary and an opponent of wholesale changes in criminal court proceedings. In 1936,
Tammany Hall nominated him to succeed the retiring Justice Edward B. La Fetra to the City Court.[13] He was elected to the City Court that November, was sworn into office that December by Surrogate
James A. Foley,[14] and was inducted into office in January 1937.[15]
In 1943, the 22 City Court Justices unanimously Byrnes Chief Justice of the City Court, which at 46 made him the youngest person to hold that position until then.[16] He was re-elected to the City Court in 1946.[17] In 1956, a
New York City Bar Association committee launched an inquiry into Byrnes' qualifications, and then asked the New York Supreme Court, Appellate Division to investigate charges of nepotism and widespread absenteeism. He admitted to having a personal relationship with three members of his staff, including a son, a sister, and his wife's cousin, although he denied the absenteeism.[18] He was re-elected to the City Court unopposed in the November election that year,[19] but in March 1957 he didn't run for re-election as Chief Justice of the City Court[20] and was succeeded to that position by Justice
Peter A. Quinn.[21] He resigned as Justice of the City Court in 1959 and was succeeded as Justice by Harry Harris.[22]
Byrnes died at
St. Vincent's Hospital on July 24, 1963. He was survived by his wife Mae and his sons James, John, and William.[1]