The Hollywood-born Dinehart was the only son of the actor Alan Dinehart and Dinehart's second wife, Actress and Journalist Mozelle Britton. In 1936, Alan Dinehart legally changed his name to Mason Alan Dinehart, the same name as his father, so that his younger son from the second marriage could be known as Mason Alan Dinehart III. This name change created confusion because the senior Dinehart's first son from his first marriage was already Alan Dinehart, Jr. (1918–1992), the former animation and voice Director for Hanna-Barbera.

Dinehart's first acting role was uncredited as "Superman" at the age of twelve in the 1948 film Superman. Dinehart was the first actor on screen to play Superman.* He played Superman as a boy diving into a haystack to recover his mother's lost ring, using his Xray vision to find it. Thereafter, he had uncredited roles as a teenager in other films. In 1954, he was cast as Ted Miller in the episode "Hot Rod" of the CBS legal drama series The Public Defender. In 1956, he played Clint Donoran in the episode "Outlaw's Son" of the syndicated television series Judge Roy Bean, starring Edgar Buchanan in the title role. In 1957, he was cast as Danny Martin in the episode "Typhoid" of another syndicated series Dr. Christian, starring MacDonald Carey.

Mason Alan Dinehart is the father of eight children from three marriages. He has two children each from his first and second marriages to Evelyn Myers (1954–1958) and Barbara Blakely (1958–1965), respectively. In 1982, he married for the third time; he and the former Miranda Gazal have four children. One of his children, Scott Dinehart, died in 2010 of an emergency hip operation.

Dinehart played the youthful Bat Masterson who is the understudy of Wyatt Earp in learning the proper techniques of frontier law enforcement. Earp rarely calls him "Bat" but "Mr. Masterson" to teach the young man maturity. In a 1956 episode "Bat Masterson Again," Earp shows young Masterson on the proper use of a pistol. During this time Masterson was elected sheriff of Ford County, Kansas, which includes the county seat of Dodge City. Bill Tilghman had been denied the right to run for sheriff again. Earp as an appointed town marshal works with an elected sheriff, and their differences in jurisdiction do not cause any problems. Bat's brother, Ed Masterson, played by Brad Johnson, formerly the deputy sheriff on the Annie Oakley television series, is shot in an ambush by drunken Cowboys, and Masterson settles the score. When Earp finally comes to Tombstone, Arizona Territory, he lacks the working relationship with Sheriff Johnny Behan that he had in Kansas with Bat Masterson.

In 1957 and 1958, he played in two youth films, as Bob Williams in The Careless Years and Joe Wilson in The Hot Angel. Other Dinehart appearances were on Sky King as Jimmy Ness in "Frogmen" and as Tex Fallon in "The Unwanted" on 26 Men. He played the character Greg in "Half a Loaf" on the syndicated western anthology series Death Valley Days, hosted in 1959 by Stanley Andrews. This role united Dinehart with western actor Bob Steele in the role of Dawson; Steele had appeared as Deputy Sam in four episodes of The Life and Legend of Wyatt Earp.

Dinehart's last roles were in 1959 and 1960, including three appearances on the CBS series The Texan, starring Rory Calhoun. Twice he played "The Brazos Kid". Dinehart played Todd Kenyon in the episode "The Swindle" of the 11-episode NBC crime drama 21 Beacon Street, a summer-replacement series starring Dennis Morgan, Joanna Barnes, and Brian Kelly. Dinehart's last screen appearance was as Bob Treadwell in the 1960 film Platinum High School.

Dinehart's Business career began c. 1960 with the Bank of America. He is now a consultant in litigation and arbitration for FEND, a company based in Los Angeles but works at times in any one of twenty-two states.