He co-starred and guest starred in numerous television Westerns during the 1950s and 1960s, including the 1965 pilot for the long-running Wild, Wild West. He also appeared in the TV series Batman as well as in many films, including First to Fight (1967), The St. Valentine's Day Massacre (1967), Bandolero! (1968), Patton (1970), Joe Kidd (1972), Against a Crooked Sky (1975), Midway (1976), Poco... Little Dog Lost (1977) and A Force of One (1979). In Patton, he had a notable scene as a wounded tank commander who briefs Patton on a fierce night battle. His television movie roles included The Bastard, Centennial and Desperate Women. Later, he would appear on Roseanne as Clint Buchanan with his OLTL costars Robert S. Woods and John Loprieno.

He is best known for his acting role as Clinton "Clint" Buchanan, husband of Victoria "Viki" Lord (played by Erika Slezak) on the ABC soap opera, One Life to Live; having originated the character in 1979, portrayed the role through 1998, with recurring stints in 1999, 2003, and 2004.

He was critically injured at California ranch on May 10, 1993, when his John Deere tractor upended, knocked him to the ground, and rolled over him. Quoted as saying that he could hear his own ribs cracking, he survived and returned to the One Life to Live set after recuperating. His co-star Erika Slezak fought not to have his role recast by another actor. His character, Clint Buchanan, was written off temporarily as having been in an airplane crash while Ritchie recuperated.

Ritchie retired in December 1998, citing One Life to Live's destruction of his character during the 1992 Viki-Clint-Sloan Carpenter love triangle story as one of the major reasons. He frequently stated, "I never got hate mail in my life until they changed Clint Buchanan". The role was eventually recast when Ritchie made it clear he was delighted to reprise his role for short stints from time to time, but had no interest in returning to acting or New York City full-time. He spent the rest of his life alone at his 60-acre (240,000 m) California ranch, with his numerous dogs and cats, and 43 horses. He purchased the ranch, named the Happy Horse Ranch, in 1980 largely due to its location near Grass Valley, which is located near the site of the annual Tevis Cup 100-mile endurance horse race. His character on One Life to Live had a ranch in Arizona with the same name, and several of Ritchie's horses were used on the show.

In late January 2009, Ritchie had surgery to implant a pacemaker. Although the surgery was a success, a blood clot traveled to his brain, resulting in a massive stroke. He died a few days later about 4:00 a.m. on January 31, 2009, aged 70. He was a good friend and co-star of Phil Carey's (Carey played Ritchie's father on OLTL), who died of lung cancer six days after Ritchie's death. Ritchie was cremated.