Boa's film credits include Man in the Middle (1964), The Adding Machine (1969), Who? (1973), The Cherry Picker (1974), The Omen (1976), Silver Bears, Superman, Carry On Emmannuelle (1978), A Touch of the Sun, The London Connection, A Nightingale Sang in Berkeley Square (1979), Silver Dream Racer (1980), Ragtime (1981), Octopussy (1983), Return to Oz (1985), and Screamers (1995).

On television, his most notable role is probably as the American guest, Mr. Hamilton, in the "Waldorf Salad" episode of the BBC sitcom Fawlty Towers. In 1977 he appeared in Come Back, Little Sheba, an episode of Laurence Olivier Presents, opposite Laurence Olivier and Joanne Woodward.

Other television credits include: The Avengers, Out of the Unknown, The Champions, The Troubleshooters, The Saint, Ace of Wands, Special Branch, The Onedin Line, Z-Cars, The New Avengers, The Professionals, The Omega Factor, Dempsey & Makepeace, Astronauts, Hart to Hart, Remington Steele, Howards' Way, the 1979 miniseries A Man Called Intrepid, the 1988 television film The Bourne Identity, Tales of the Unexpected, As Time Goes By, Road to Avonlea, Kavanagh QC, Bulman and Warship.

Boa died from cancer on 17 April 2004 in Surrey, and had lived in England since the 1960s.