There were six lead changes in the opening three minutes, then Ohio State used an 11-2 run to open up an 18-10 advantage.
Cat-quick MSU guard Alexis Rack canned back-to-back treys to quickly cut into the OSU lead, and though Johnson stopped the mini-run at six by putting back a Jantel Lavender miss for a layup, Rack's hot shooting turned out to be a harbinger of what was to come.
Ohio State managed to re-establish an eight-point lead at 27-19 with 8:56, but then Rack scored consecutive MSU baskets with slicing drives to start a half-ending 27-7 MSU run that sent the Buckeyes reeling into the locker room.
The favorites trailed 46-34 at the half and never recovered.
The closest the Buckeyes got in the last 20 minutes was nine on two occasions.
The latter came when Johnson hit another three-pointer at the 8:57 mark. That came after Ohio State's Shavelle Little won a stare down with Rack in the previous possession. The two-time Big Ten Defensive Player of the Year stayed in front of Rack when the Bulldog faked left then right, and Little poked the ball away when she chose to attack left. Rack scrambled after the loose ball but knocked it out of bounds, prompting Little to let out a triumphant scream.
"I think it was a big stop to give us a little bit of energy," Little said. "We didn't have enough energy. We were trading baskets with them, and we couldn't do that when we were down."
Johnson's three came on the ensuing possession, but Rack answered with a three in the face of Prahalis, then Lavender was called for traveling at the other end.
That was the first of three straight turnovers Ohio State turnovers and helped fuel what was a 9-0 run when counting Rack's trey as the starter.
That she would can light the fuse for the final combustion of the Buckeyes' season was appropriate, because it was Rack's night without a doubt.
She finished with a game-high 30 points on 10-for-18 shooting that included 6 three-pointers on 12 tries.
"I was just taking what they gave me," the senior point guard said. "Once I get hot, the ball was just going in every time."
Ohio State could not handle her quickness off the dribble or the expedience of her release when she came off a ball screen and found daylight behind the three-point arc, something that seemed to happen with regularity.
"I think we had a great scouting report on Alexis Rack," Lavender said. "I don't know why we were so far off her shooting wise. We knew she could shoot the deep three. We knew she could drive. She capitalized on us not being aggressive on the screens. She took advantage of that and was very effective."
Meanwhile, Ohio State's two stars both had nights to forget.
Lavender finished with 17 points, but many of them came after the Buckeyes had been put away. She was not a factor at either end for Ohio State early in the game. The Buckeyes could not get her the ball in any position to score, and the 6-4 junior was powerless to stop a deadly turnaround fadeaway jump shot 6-5 Chanel Mokango nailed over her on the low right block three times during the Bulldogs' big run to close the second half.
The MSU center outscored the three-time Big Ten Player of the Year 11-4 in the opening half and looked nothing like the tentative player Lavender abused a year ago when the Buckeyes knocked the Bulldogs out of the tournament with a 68-64 decision in the same round.
Mokango was just 1 for 11 from the field with two points that night, but this time around made 9 of 11 field goals and finished with 19 points while Armelie Lumanu had 17 and Mary Kathryn Govero added 15.
As for the Buckeyes' other star, Prahalis exited the game at the 4:22 mark after picking up a technical foul when she protested the ball being awarded to the Bulldogs when it looked as if she was bumped out of bounds.
The technical also counted as her fifth personal, and she finished with 14 points, six assists and 10 turnovers.
She made only 5 of 12 shots and rarely looked comfortable on the floor.
"When Sammy started to struggle, we got tentative because we're not used to seeing that," Ohio State head coach Jim Foster said.
Sarah Schulze, who hit a trio of threes as her team built its early lead, finished with 12 points and said the Buckeyes did not play like themselves.
"We let them just kind of control how we were playing," she said. "It was like we were scared of them when we were on offense. It was just lazy passes. I don't know. It wasn't our night.
Asked how a team could be scared of another at this point in the season, she shook her head.
"Honestly, I don't know. Whenever they started getting up in us, we just backed off. I don't know why we did that tonight, but we did."
The Bulldogs outscored Ohio State by an astonishing 20-0 off turnovers in the opening half and finished the game with a 37-9 advantage in that category.
"We felt like our defense could be a deciding factor," said Mississippi State coach Sharon Fanning-Otis, whose team forced 24 turnovers and committed 11.
Ohio State lost to a team with a lower seed for the fifth time in six NCAA tournaments, although the senior class finished with a school-record 110 victories in their four years.
MSU advances to a Sweet 16 matchup with No. 3 Florida State on Sunday at the University of Dayton Arena in Dayton.