Numerous roles followed, and by his mid-teens, Halpin had appeared on many of the major TV series of the day: Armstrong Circle Theatre, The United States Steel Hour, Kraft Television Theatre, Hallmark Hall of Fame, The Phil Silvers Show, Harbormaster, The Defenders, Route 66, Naked City, The Everglades, and had a recurring role for six months on the soap opera Young Doctor Malone.

He made his Broadway debut in Take Me Along starring Jackie Gleason, and appeared in Sunrise at Campobello, and with Mary Martin in both Annie Get Your Gun and Peter Pan.

Kelly and Halpin kept the same roles for the television series that began filming in the summer of 1964, adding younger brother Bud, played by Tommy Norden.

By the time filming of the TV series commenced, Halpin had become an expert skin and scuba diver and exhibited an easy athleticism that enabled him to perform many of his own stunts in and below the water including a number of dangerous scenes involving sharks.

After Flipper ended, Halpin appeared in feature films, including as Stu MacRae (teen son of Richard Greene's starring character) in Ivan Tors' Island of the Lost (1967), as Bo (student radical who befriends a teenage girl travelling on a European bus tour) in the comedy If It's Tuesday, This Must Be Belgium (1969), as Keith (First Mate on a shabby vessel chartered for a tour that stumbles on 'living dead' Nazis) in iconic niche horror movie Shock Waves (1977) and as Ken Wilson in Flipper co-creator Ricou Browning's Mr. No Legs (1979).

Halpin's guest appearances on television in the years shortly after Flipper were; as Kenny Carter, Jr., in the Carl Betz series Judd, for the Defense (1968), as a celebrity contestant on The Dating Game (1968), as Ben Cabot, Jr., in Bracken's World (1969), as Greg in Ivan Tors' Primus (1972), and as Eric Bates in Caribe (1975).

[7] Following an acting career that spanned three decades, Halpin began working as a stuntman, marine coordinator, diver, and speedboat pilot for such feature films as Never Say Never Again, Porky's Revenge!, Flight of the Navigator and Speed 2: Cruise Control as well as for the television series Miami Vice.

[8][9] He also continued to make cameo appearances, most notably, on the television series Key West, Miami Vice and in the 1996 feature film remake, Flipper, starring a 15-year-old Elijah Wood as Sandy Ricks.