Norwegian Boasson Hagen has been on the wrong side of two photo finishes so far in this Tour but he made certain of victory in Salon-de-Provence as he attacked out of the breakaway with three kilometres to go and raced solo to the line.
Nikias Arndt did his best to keep up but the Team Sunweb man came home in second place, five seconds behind.
That all happened a good 12 and a half minutes before the peloton crossed the line, with the contenders taking it relatively easy after two long days in the Alps.
Team Sky’s Froome leads by 23 seconds from Frenchman Romain Bardet and 29 seconds from Colombian Rigoberto Uran going into Saturday’s penultimate stage, a 22.5km time trial starting and finishing in Marseille’s Velodrome stadium.
Team Dimension Data’s Boasson Hagen was denied by just six millimetres – or 0.0003 seconds – when Marcel Kittel won stage seven to Nuits-Saint-Georges in the first week of the Tour.
He was then beaten again by Michael Matthews on stage 16, a victory that would have been all the sweeter for his South African-based team as it would have come on Mandela Day.
Add in two third places along the way, and he was due a victory.
“I’m really happy,” Boasson Hagen said. “It’s been really close many times, but today I managed to make an attack so we didn’t have to do a photo finish.
“I’ve been getting confidence by seeing I could be so close so many times. It’s always bitter when you’re not winning but now I’m so happy.”
A day after the final mountain test, the organisers threw the longest stage of the entire Tour at the riders as they rode 222.5km from Embrun.
Although this stage was listed as flat, Sky were eager to avoid it coming to a sprint finish given the technical nature of the final three kilometres – with three sharp bends perhaps inviting a crash if the race was on.
As such, they allowed a 20-man break – also including another ex-Sky man in the shape of Ben Swift – to go almost 10 minutes up the road.
It began to fracture inside the final 20km, with Boasson Hagen part of a nine-man group that left Swift and the others behind.
At a roundabout with four kilometres to go, Arndt and Boasson Hagen went down the right of a roundabout while the others went left, and that was the moment the duo went clear.
“I’d been looking at the book before so I knew it was shorter on the right,” Boasson Hagen said. “Nikias knew that as well. When he made an attack I followed him, then I could counter-attack and he couldn’t follow.”
It was Boasson Hagen’s third career Tour stage win, with the other two coming in Sky colours in 2011.
Froome is now firmly odds on for a fourth Tour title with Saturday’s time trial expected to see him gain further time on his rivals.
The 32-year-old certainly looked in relaxed mood as the peloton cruised along, and he even took time to high-five famed Tour fan Didi the Devil along the way.
Sir Chris Hoy has warned British cycling members they risk bringing an end to the sport’s era of success if they fail to back governance reforms at an extraordinary general meeting of its national councils today.
British Cycling is one of the national governing bodies which must comply with sports minister Tracey Crouch’s code to promote independence and diversity by October – or face the prospect of losing more than £40million of UK Sport and Sport England funding.
A 75 per cent majority vote is required for the reforms to go through but three of 10 regions have rejected the proposals at regional mandate meetings, prompting six-time Olympic champion Hoy to pen an open letter to those responsible for making the decision, urging them to embrace the reforms for the sake of the sport.