Robertson, who was best known for his role of special agent Jim Hardie in the NBC television series TALES OF WELLS FARGO from 1957-1962, died on Tuesday, the hospital said.
The "Sitting Bull" star had been in poor health for about two years and had a cancer diagnosis last week, his niece, Nancy Love Robertson, told The Oklahoman newspaper.
Born Dayle Lymoine Robertson in Harrah, Oklahoma, in 1923, the actor attracted the attention of Hollywood agents after a Los Angeles photographer posted his photo in a display window.
Robertson, who served in Europe and Africa during World War Two, starred in 60 films and television shows over his five-decades acting career, starting out with roles in 1950s Westerns such as Devil's Canyon and Dakota Incident.
His soap opera roles included Walter Lankershim in DYNASTY and Frank Crutcher in DALLAS.
The actor was inducted in 1983 into the National Cowboy and Western Heritage Museum's Western Performers Gallery in Oklahoma City, alongside some of Hollywood's most famous on-screen cowboys including John Wayne and Roy Rogers.
Robertson is survived by his wife, Susan, and two daughters.