He played the roving investigator Jim Hardie in the television series Tales of Wells Fargo and railroad owner Ben Calhoun in Iron Horse.

During World War II, he was commissioned through Officer Candidate School, and served in the U.S. Army's 322nd Combat Engineer Battalion of the 97th Infantry Division in Europe.

Two other uncredited appearances led to featured roles in two Randolph Scott Westerns: Fighting Man of the Plains (1949), where he played Jesse James, and The Cariboo Trail (1950).

He returned to Fox for City of Bad Men (1953) with Crain; The Silver Whip (1954) with Rory Calhoun and Robert Wagner; and The Gambler from Natchez (1954) with Debra Paget.

[10][11] Robertson also did the narration for Tales of Wells Fargo through which he often presented his own commentary on matters of law, morality, and common sense.

In its cover story on television Westerns, published March 30, 1959, Time reported Robertson was 6 feet tall, weighed 180 pounds, and measured 42–34–34.

[12] In 1962, he similarly appeared and sang a perfect rendition of "High Noon" on the short-lived Western comedy and variety series The Roy Rogers and Dale Evans Show.

Robertson created United Screen Arts in 1965[15] which released two of his films, The Man from Button Willow (1965, animated) that he did the voice for and The One Eyed Soldiers (1966) which he starred in.

In the 1966–67 season, Robertson starred in Scalplock another television pilot released as a movie that became Iron Horse, in which his character wins an incomplete railroad line in a poker game and then decides to manage the company.

[3] In 1968, he succeeded Robert Taylor as the host of Death Valley Days, a role formerly held by Stanley Andrews and future U.S. President Ronald Reagan.

In rebroadcasts, Death Valley Days (often known as Trails West at the time), featured Ray Milland in the role of revised host.

In 1983, Robertson made Big John, another television pilot, where he played a Georgia sheriff who becomes a New York Police Department detective.

In his later years, Robertson and his wife, Susan Robbins, who married in 1980, lived on his ranch in Yukon, Oklahoma, where it was reported he owned 235 horses at one time, with five mares foaling grand champions.

Due to his declining health, he relocated to the San Diego area in what would be his final months, passing away at Scripps Memorial Hospital in La Jolla, California, on February 27, 2013, from lung cancer and pneumonia.