Halpin's career began when a music Teacher, impressed by Halpin's "all-American" look, encouraged him to try acting. In 1955 he co-starred with Natalie Wood in an episode of Studio One entitled "Miracle at Potter's Farm". 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.

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' 1962 (released 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 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, and young widowed father to Sandy (although in reality Halpin was only sixteen years younger than Kelly). Kelly and Halpin kept the same roles for the Flipper (1964 TV series) that commenced filming in the summer of 1964 (when Halpin was 17) adding younger brother Bud, played by Tommy Norden when Norden was 11.

The Flipper (1964 TV series) ran for eighty-eight episodes from 1964 to 1967 (with Halpin appearing in all but three episodes) and is still in syndication. It was filmed in the park and waters around Key Biscayne, Florida, at the Ivan Tors (now Greenwich) Studios and the Miami Seaquarium, both in Miami. The series performed strongly in the hotly contested Saturday night TV slot, rating in the Top 25 of all TV shows in its debut 1964-65 season. The series made Halpin a teen idol, especially among adolescent viewers. He was often featured in such magazines as Bravo, Teen Life, 16 Magazine, and the earliest issues of Tiger Beat. On the basis of his appearances in the original Flipper feature films, Halpin was a guest 'contestant' on the CBS panel show To Tell the Truth just prior to the beginning of filming of the Flipper (1964 TV series), on March 30, 1964. He appeared on the show again just prior to the filming of the second season, on April 15, 1965.

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 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). TV guest appearances in the years shortly after Flipper include 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).

He appeared in Death Valley Days in 1968, where he played Sandy King, a young member of the Curly Bill Brocius gang, who is befriended by a United States Army lieutenant, played by Sam Melville.

Halpin lives on the west coast of Florida with his wife, Deborah. He has three sons, Kyle Austin Halpin (born October 1980), Blair Luke Halpin (born December 1982), and Courtney Luke Halpin (born April 1990). In 2015 it was reported that Halpin was suffering from Stage IV head and neck cancer. A website to assist in his care has been established. In June 2016 a family friend announced the cancer was in remission but that he was suffering from the early stages of Alzheimer's disease.

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. 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.