It was the first time in training camp Manziel had a first-team rep, and he spent the entire session with the starters, two days after the Browns' intrasquad scrimmage.
Manziel is competing with Brian Hoyer for the starting job.
"Just part of the plan that we've had," Pettine said. "We'll go back and forth."
Pettine said the back-and-forth will be day-to-day yet did not specify which days. He also declined to say Hoyer would start the team's preseason opener Saturday at Detroit but said there was a "more than reasonable chance" it would be Hoyer.
Manziel and Hoyer were not available to the media on Monday.
Pettine said the plan all along -- although not written in stone -- was to give Manziel the first-team reps at this practice.
"It was not a result of what happened in the scrimmage," he said.
Manziel's numbers were not that impressive in the scrimmage, but the way he played was. He guided the offense on a 16-play drive and threw a touchdown pass to tight end Gary Barnidge that officials incorrectly ruled incomplete. Hoyer had a higher completion percentage but no touchdowns.
"They played to their skill set," Pettine said. "I thought Johnny did a good job making plays with his feet, keeping plays alive. And Brian showed some poise in the pocket, stepped into some throws. Let's not forget he's coming off a knee [injury]."
Manziel didn't get a lot done in the practice with the first team. His first throw was a deep completion that was underthrown that Joe Haden somehow did not break up. Soon after, Haden tipped a pass for Josh Gordon that Donte Whitner intercepted.
Hoyer led the offense to a field goal and touchdown in two-minute drills; Manziel did not score.
"We say we were going to allow the quarterbacks to compete," Pettine said. "At some point you have to mix up the supporting cast a little bit. It's all part of it."
Pettine smiled when it was pointed out to him that this one day of practice will cause a lot of hoopla and chatter, given the rookie involved.
"That's great," Pettine said. "It's all part of it."