Following a series of jobs ranging from clothing salesman to short-order cook, Rivera enrolled at Brooklyn Law School in 1966. As a law student, he held internships with the New York County District Attorney under legendary crime-fighter Frank Hogan and Harlem Assertion of Rights (a community-based provider of legal services) before receiving his J.D. near the top of his class in 1969. He then held a Reginald Heber Smith Fellowship in poverty law at the University of Pennsylvania Law School in the summer of 1969 before being admitted to the New York State Bar later that year.