Garrett has led the Cowboys to just three winning seasons and two playoff appearances in that time while posting a 67-53 overall record.
"I think you've got to be able to give a head coach time," Johnston said during an appearance on DFW's Fox4, per the Dallas Morning News. "It's one of the great things John Madden said, and this was about the San Francisco 49ers when they moved away from Steve Mariucci, but they had no plan after that. Sometimes you do that and all of a sudden you find yourself in a decade of mediocrity, and in San Francisco's case, not even mediocrity. That was a tough decade. They made a change for change's sake and nobody was there.
"Jason Garrett, in my opinion, moving forward gives Dallas the best opportunity. They've got that window right now with a quarterback [Dak Prescott] and a very talented running back [Ezekiel Elliott] on rookie contracts. To make a change right now, that would be sabotage to that organization. Keep everything in place, take a run with this window that's there."
Johnston spent 11 seasons with the Cowboys before serving as a longtime color analyst on FOX broadcasts for NFL games.
In May, Owens said it was "mind-boggling" that Dallas opted to bring Garrett back after the team finished with a 9-7 mark last season and missed the playoffs.
"It all boils down to players being the scapegoat for [Garrett's] inability to lead the team as he should," Owens said in May. "For me it's mind-boggling. I don't understand. And I think Jerry [Jones] -- again he's the owner at the end of the day, he has to feel good with himself about the decisions -- but I just don't understand why this guy still has a job."