Santoni was born in New York City of Franco-Spanish descent.

His first notable role was an uncredited appearance in the 1964 film The Pawnbroker, starring Rod Steiger where he played a junkie attempting to sell a radio to the title character. His other well-known film roles include Inspector "Chico" González in the 1971 classic Dirty Harry and as Ramon Herrera in the 1983 hit drama Bad Boys. Around that time he also made guest appearances on television shows such as Barnaby Jones, Hardcastle and McCormick, Hill Street Blues and Midnight Caller.

Santoni continues to work in film and television to the present day. Some of his more memorable characterizations include "Poppy", the unhygienic, incontinent, anti-abortion restaurateur on TV's Seinfeld, and as "Captain Carlos Rodríguez" in Carl Reiner's comedy film, Dead Men Don't Wear Plaid.