The move, made on the final hill of the day, had no effect on the general classification, however, with Marc Hirschi (UAE Team Emirates) securing the yellow jersey after an aggressive finale in Luxembourg.
Alex Aranburu (Movistar) and Franck Bonnamour (AG2R Citroën) rounded out the podium from the late attack, which was made 7km from the line.
"It's so hard to win this kind of bike race," Johannessen said after the stage. "The level in this race has been incredibly high, so, to be honest, I thought it would be impossible for us to win.
"It was a really hard start and a hard day, and I know that after a hard day, my legs could still be good. So, we just continued to believe. We decided to go for it, and I'm really happy to get the win today.
"Of course, I thought it would be impossible to get away, but sometimes, when you come from the back, and the group stops, then you can go. If nobody follows straight away, it can be hard for them. I knew I wanted a victory this season, so I knew that if I got a gap, I could go full gas and probably get to the line. In the end, it worked out."
UAE Team Emirates had kept control at the head of the peloton, neutralising dangerous moves and helping Hirschi take two bonus seconds two laps out.
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Attacks flew at the front on the hilly closing circuit, but nothing stuck until a sizeable group went clear on the final lap.
Johannessen was in there along with the likes of Bob Jungels (Bora-Hansgrohe), Julian Alaphilippe (Soudal-QuickStep), and Natnael Tesfatsion (Lidl-Trek), though nobody in the group posed any GC threat.
As a result, the 13-man attack got clear and were allowed to battle it out for the stage win. It was the Norwegian who eventually prevailed, taking a flier on the last uphill rise to steal a march on his rivals and get a gap.
He'd solo home over the final 2km to win by eight seconds from Aranburu, while Hirschi finished safely in the peloton at 22 seconds, his title in the bag. He won the race by three seconds from his UAE Team Emirates teammate Brandon McNulty, while Ben Healy (EF Education-EasyPost) rounded out the overall podium at five seconds down.
"It was very stressful," Hirschi said of the stage. "A big break went at the start, and it was very good for us. But Jumbo-Visma, AG2R-Citroën, Arkéa-Samsic, and Alpecin-Deceuninck weren't happy, and it was full gas all day to bring back the breakaway. Then, on the steep climbs, I just watched out for Ben because he was so close in seconds.
"In the end, it was good that a small group made it to the finish, so I was safe with the bonus seconds. In the end, I had five seconds on Ben, and it was enough. My legs were tired today, so I was happy that the break could take the bonus seconds.
"I had a very strong team with me, so basically, I just knew I needed to watch Ben. I still had Diego Ulissi, Felix Großschartner, and Brandon McNulty, in case, so that made me kind of relaxed. I knew that these guys could bring back many guys. I had to watch out for Ben, and luckily, I could follow him."
How it unfolded
The 177.2km stage was fast-paced and aggressive from the start, with the break of the day taking over 60km to form as the likes of Jumbo-Visma and AG2R Citroën chased down move after move during the early hills.
Several strong riders, including Magnus Cort (EF Education-EasyPost), Andrea Bagioli (Soudal-QuickStep), and Marco Haller (Bora-Hansgrohe) made the eventual move, while stage 4 winner Victor Campenaerts (Lotto-Dstny) and Julian Alaphilippe (Soudal-QuickStep) bridged across at 110km to go to make it 12 out front.
Back in the peloton, the same mix of teams continued to work to hold the gap low, with Jumbo-Visma particularly active, having missed the attack. The gap to the break held around the minute mark at the mid-point of the day before attacks started at the front inside the final 60km.
Bagioli would eventually go clear alone heading into the last 50km of the day, the Italian pushing on alone even as the remainder of the break was eventually caught. He'd persevere until the final circuit but would only last until the hill at 26km to go, prompting fresh counters.
At that point, Richard Carapaz (EF Education-EasyPost) launched his big move, pushing on solo and leading across the line at two laps to go just after Hirschi snatched two bonus seconds at the line.
He held a small gap to 10 seconds, one that was bridged by Bingoal WB rider Alexis Guerin at 18km to go. The pair built a 20-second lead, but with the pace upping behind, they'd only last until the 10km marker inside the final lap.
Multiple attackers made their move 7km out, forming the victorious move after Ewen Costiou (Arkéa-Samsic) kicked off the new wave of aggression. However, luckily for UAE Team Emirates, none of Hirschi's main rivals were out front, leaving the new break free to contest the finish.
More attacks at the front came as the finish line edged nearer, though it was Johannessen who had the nous and strength to go clear, jumping from the rear of the group to take the rest by surprise before soloing home for the fourth victory of his pro career.
Dani Ostanek is Senior News Writer at Cyclingnews, having joined in 2017 as a freelance contributor, later being hired full-time. Her favourite races include the Tour de France Femmes, Paris-Roubaix, Tro-Bro Léon, and cycling's fourth Grand Tour, the Volta a Portugal.
