Manetti studied acting in Chicago with the Ted Liss Players. His first television role was as a young detective in
Jack Webb's
Chase (1973–1974). He would go on to play pilot Bobby Boyle in
Baa Baa Black Sheep (1976–1978) with
Robert Conrad. He followed this up with a role as a bookie on the short-lived NBC series
The Duke (1979), which also featured Conrad in the title role. His signature role on
Magnum, P.I. lasted for the entire eight-year run of the series (1980–1988). He also had co-starring roles in twenty-five feature films, and guest-starred on many other television shows, including
The Rockford Files,
Emergency!,
Starsky & Hutch,
Tenspeed and Brown Shoe,
Fantasy Island,
Battlestar Galactica,
Renegade,
Quantum Leap,
JAG, and
Walker, Texas Ranger. He also had a minor role in the 1993 film
CIA II: Target Alexa. It was announced on January 24, 2013 that Manetti had joined CBS' reboot of
Hawaii Five-0 in the recurring role of Nicky "The Kid" Demarco. The character is described as "a local lounge legend ... [who], back in the day, was mentored by the one and only Frank Sinatra." In October 2019, he appeared in the reboot of
Magnum P.I., playing the same character. He also reunited with Tom Selleck on
Blue Bloods as troubled retired police sergeant "Sam Velucci". ==Other==