Although this burly character actor has divided much of his time between film and TV, often playing policemen, Hallahan started his career on the stage. After four years in the Navy and several more in college (at Rutgers and Temple), Hallahan joined San Francisco's American Conservatory Theater (ACT) in the early 1970s and appeared in more than 40 productions in over five years, including "One Flew Over the Cuckoo's Nest", "Equus", "Peer Gynt", "Man and Superman", and "The Threepenny Opera". In 1976, he traveled with ACT on its tour of the Soviet Union and performed in O'Neill's "Desire Under the Elms" and Thornton Wilder's "The Matchmaker".
By 1977, Hallahan had landed in Hollywood, where he made guest appearances on such series as "Lou Grant", "Trapper John" and "Hill Street Blues". His first recurring role was in the short-lived "The Paper Chase" (CBS, 1978-79). He started his TV-movie career with a small role in "Terror Out of the Sky" (CBS, 1978) and played meaty supporting roles in numerous TV-movies and miniseries over the next two decades. Some highlights include co-starring in "Mickey Spillane's 'Margin for Murder'" (CBS, 1981), appearing as the young hero's father in the drama "A Winner Never Quits" (ABC, 1986), portraying Joseph McCarthy in the biopic "J. Edgar Hoover" (Showtime, 1987), essaying detectives in the thrillers "Cast a Deadly Spell" (HBO, 1991) and "When Love Kills: The Seduction of John Hearn" (CBS, 1993), and playing a pilot who witnesses a mystery at "Roswell" (Showtime, 1994).
Hallahan also had roles in several TV series. After "The Paper Chase", he played a captain in the police series "Hunter" (NBC, 1986-91), appeared in the new wave fantasy miniseries "Wild Palms" (ABC, 1993), and played Brett Butler's boss during the 1993-94 season of "Grace Under Fire" (ABC). In 1995, he reunited with his "Hunter" co-stars for a TV-movie, "The Return of Hunter" (NBC).
Films have also provided Hallahan with good character parts. He made his bow in the 1979 horror flick "Nightwing", then had small roles in "Going in Style" (also 1979), "Hide in Plain Sight" (1980), the grisly "The Thing" (1982) and the notorious Vic Morrow segment of "Twilight Zone-The Movie" (1983). With few exceptions, Hallahan's movies were high-budge, though not always high quality. He had a supporting role in "Silkwood" (1983), played Matthew Modine's coach in "Vision Quest" (1985) and appeared in the Clint Eastwood western "Pale Rider" (also 1985). There were roles as cops in the comedies "Fatal Beauty" (1987) and "Dave" (1993), and he was a doctor in the 1993 courtroom drama "Body of Evidence" (in which Madonna's body was much in evidence).