Halpin's most famous role came when at age 15 he was picked to play the 12-year-old Sandy Ricks in producer Ivan Tors' 1963 feature film "Flipper" (filmed in the Florida Keys and Miami), starring alongside Chuck Connors, who played Sandy's father and fisherman Porter Ricks. The successful film spawned a sequel, "Flipper's New Adventure" (filmed mainly in the Bahamas), released in 1964, and with new co-star Brian Kelly as Porter Ricks, a trainee Park Ranger who, after his training, is assigned to the fictitious Coral Key Park. Kelly played a widowed father to Sandy (although in reality Halpin was only sixteen years younger than Kelly). 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.