Johnson's goal came after Alex Pietrangelo was called for delay of game after he shot the puck over the glass.
"We did that the other night," Vegas coach Bruce Cassidy said. "That's on us a little bit, right? You've got to manage it a little better with time and score. Situational hockey. Maybe it's just more of a lower rim (around the boards). Veteran guys, you've just got to manage it better in those situations."
Cole Guttman scored, and Mrazek made 34 saves for the Blackhawks (19-32-5), who won their third straight.
Jack Eichel and Keegan Kolesar scored for the Golden Knights (34-18-5), who had won five in a row. Laurent Brossoit, who was playing his first NHL game since March 15, 2022, made 37 saves.
"He was very good, he gave us a chance to win," Cassidy said. "The goals that beat him were good shots. He made some good saves when we broke down in front of him, especially early on, getting tested there.
"Guys were getting behind us. We didn't play a very respectful game at all. He was there for us when we broke down in basic system coverages. I liked his game."
Brossoit earned the start after Adin Hill sustained a minor injury in a 5-4 win against the Tampa Bay Lightning on Saturday.
"I try not to have too many expectations," Brossoit said. "I just wanted to keep it simple, play my game. For the most part, I felt pretty good. A few jitters in the first period, but I think overall it was a net positive for how I felt. There are plenty of things I can sharpen up.
"It's a lot of touches, 39 shots. It's nice to see the puck that many times the first game back. Yeah, it did make things a little easier. It was huge. Having a lot of shot volume is big for the confidence, just getting used to the pace of play again. But at the end of the day, we didn't get the result we wanted, so I'm still looking to get better."
Patrick Kane appeared to have scored for Chicago with a slap shot on a breakaway in the final second of overtime, but a video review determined that time had expired.
"Yeah, that was exciting," Richardson said. "I thought it was in because the feed that we had, it looked like it was like .01 still on it, but I think it's a TV feed, so it's not the actual clock feed. We just had to beat them twice, I guess."