Horowitz was born in New York City in 1936. He received his A.B. degree from Rutgers University in 1958 and his LL.B. degree from Columbia University School of Law in 1961. He served as law clerk to Milton B. Conford (New Jersey Superior Court Appellate Division) from 1961 to 1962. In 1963 he was named Assistant U.S. Attorney at the U.S. Attorney's Office for the District of New Jersey. He was Chief of the Criminal Division in 1966 and First Assistant in 1968. In 1969 he was Acting U.S. Attorney until the appointment of Frederick B. Lacey by President Richard Nixon.U.S. Attorney's Office District of New Jersey, A Rich History of Service From 1969 to 1970 he served as Special Deputy Attorney General of New Jersey.
