Robins striker Aaron Wilbraham, who earlier had been twice denied by the impressive Johnstone, cancelled out Jermaine Beckford’s strike as the points were shared between the top two in the table.
And Cotterill, whose side remain eight points clear of Preston at the summit and 13 ahead of third-placed MK Dons, praised the North End stopper.
“I think we deserved to win the game in the end. Their goalkeeper got man of the match and that tells its own tale,” said Cotterill.
“He’s pulled off some magnificent saves from Aaron Wilbraham and Kieran Agard. How he’s kept those out I’ll never know and I thought he was magnificent.
“It was a really good game. We wanted to win, as we have set up to do so every game this season. You can get caught out being too cavalier but that’s how we’ve played all season and we’re not going to change now.
“We pinned them into their 18-yard box for a long time and then from one clearance we’ve not defended it properly. We’re disappointed with how we did for the goal but we’ve bounced back well.
“It’s Aaron Wilbraham’s 20th goal of the season in all competitions and we’re really pleased for him.”
Wilbraham’s glancing header, from Joe Bryan’s cross, forced Johnstone into a great reflex save before another header from the City forward clattered the underside of the crossbar after 49 minutes, courtesy of Johnstone’s fingertip save.
Beckford then steered Paul Gallagher’s low shot in from close range after 59 minutes to put the hosts in front.
But the lead proved short-lived, with Aden Flint heading across goal for Wilbraham to nod in four minutes later.
“I think you saw a game between two teams desperate to win,” said Preston manager Simon Grayson.
“Over the course of the game it was probably a fair result. There were chances in both boxes and while they’ve created the better ones with the saves Sam’s had to make, we did have chances of our own.
“Four points off the league leaders is a good return but we’d have loved to have taken six. It was a tough game, with two teams cancelling each other out.
“It was disappointing not to push on after taking the lead, but our players have stood up and been counted against a physical and energetic team who are top of the table for a reason.
“It was a big performance by Sam Johnstone. He’s contributed to another great performance by the team. You don’t want him to be busy in an ideal but when he was called upon he’s made all the right decisions.”