Franciscus was born in Clayton, Missouri, to Lorraine (née Grover) and John Allen Franciscus, who was killed in action during World War II. In 1957, Franciscus received a Bachelor of Arts degree in English and theatre arts from Yale University in New Haven, Connecticut, from which he graduated magna cum laude. He was a classmate of Dick Cavett and Bill Hinnant.  His first major role was as Detective Jim Halloran in the half-hour version of ABC’s The Naked City television series. Franciscus guest starred on the CBS military sitcom/drama Hennesey, starring Jackie Cooper, and on the NBC drama about family conflicts in the American Civil War entitled The Americans. CBS soon cast him in the lead in the 13-week series The Investigators, which aired from October 5 to December 28, 1961. He played the insurance investigator Russ Andrews, with James Philbrook as a co-star. Franciscus was also cast in the role of Tom Grover in the 1961 episode “The Empty Heart” of the CBS anthology series, The DuPont Show with June Allyson. He performed in many feature films and television programs throughout the 1960s and 1970s, preceded by a minor role in an episode of The Twilight Zone titled Judgment Night in 1959, and a major role in an episode of Alfred Hitchcock Presents: Summer Shade in 1961.

He starred in I Passed for White, in 1960, and in 1963, he appeared as Mike Norris in the episode “Hang By One Hand” on the NBC medical drama about psychiatry, The Eleventh Hour, Combat! and Miracle of the White Stallions. Franciscus may be best-remembered for his title roles in NBC’s Mr. Novak (1963–65) and ABC’s Longstreet (1971–72), and for his vocal performance in the big-screen version of Jonathan Livingston Seagull (1973). Along with Lloyd Bridges, he served as host as the syndicated real-life adventure series Waterworld, which aired from 1972 to 1975. In 1977, he starred in his fifth television series, the short-lived Hunter (as a secret agent).  He was also frequently seen in feature films of the 1960s and 1970s such as Youngblood Hawke, Snow Treasure, The Amazing Dobermans, Marooned, Beneath the Planet of the Apes, City on Fire, When Time Ran Out, The Valley of Gwangi, and The Greek Tycoon. Over the years, Franciscus found film work with Italian cinema. In 1971, he accepted the lead role in Dario Argento’s second film, The Cat o’ Nine Tails. In 1979, he appeared in Antonio Margheriti’s Killer Fish, and in 1980 he starred in director Enzo G. Castellari’s Jaws-inspired Great White (aka The Last Shark). He continued appearing in roles on the screen and television. When less important roles were offered, Franciscus turned to writing screenplays and producing. In 1991, the year of his death, he worked as an associate producer and screenwriter on the movie 29th Street starring Anthony LaPaglia and Danny Aiello; it was his final project.  He was the voice of Jonathan Livingston Seagull, in the movie of the same name, written by Richard Bach. The movie was released in October 23, 1973.