Lonny Chapman was born in Tulsa, Oklahoma, but lived thereafter in Joplin, Missouri. He graduated from Joplin High School and, in 1940, from Joplin Junior College, the predecessor institution of Missouri Southern. He enlisted in the United States Marine Corps served in the South Pacific during World War II. After the war, Chapman graduated from the University of Oklahoma at Norman and then moved to New York City, where he landed the role of Turk in Come Back, Little Sheba. After moving to California, he appeared in Young at Heart (1954), East of Eden (1955), Baby Doll (1956), and The Birds (1963). He also had a starring role in the short-lived 1965 series For the People. His other film roles included appearances in A Covenant with Death (1967), Hour of the Gun (1967), The Stalking Moon (1968), Take the Money and Run (1969), The Reivers (1969), I Walk the Line (1970), Welcome Home, Soldier Boys (1971), The Cowboys (1972), Run, Cougar, Run (1972), Where the Red Fern Grows (1974), The Witch Who Came from the Sea (1976), Moving Violation (1976), The Bad News Bears Go to Japan (1978), Norma Rae (1979), Hanging by a Thread (1979), When Time Ran Out (1980), Running Scared (1980), Amy (1981), The Border (1982), Cave-In! (1983), 52 Pick-Up (1986), Reindeer Games (2000), and The Hunted (2003).

In 1972, Lonny Chapman founded the Lonny Chapman Group Repertory Theatre in Los Angeles and was named artistic director, a title he held until his death. Under his direction, the nonprofit 99-seat theater staged more than 350 productions and at least 45 premieres of original works. He was also a playwright; his works The Buffalo Skinner and Cry of the Raindrop were both produced Off-Broadway. In the fall of 2005, Chapman was named “Outstanding Alumnus” at Missouri Southern State University in Joplin. Lonny Chapman died in an area care facility of complications from heart disease, and was survived by his wife of 63 years, Erma (née Dean), and their son, Wyley.