Boasson Hagen, a class act who won two stages for Team Sky in 2011, but who this year endured two runners-up finishes and two third places, then outclassed Arndt to take the victory by a comfortable 5 seconds.
Winless this year and with doubters starting to circling Sunday's largely ceremonial last stage into Paris will now serve to remind the cycling world of Froome's enduring power.
The Brit finished six seconds behind stage victor Maciej Bodnar on Saturday but yhe 32-year-old took time out of all his major yellow jersey rivals on a 22.5km individual time trial.
"It's just an unbelievable feeling", Froome said.
The Brit has now won four of the last five editions of the most prestigious Grand Tour - including each of the last three - and was all smiles as he crossed the line with his team-mates in Paris.
AG2R La Mondiale's Bardet, who slipped to third behind Cannondale-Drapac's Rigoberto Uran on Saturday, did push Froome to the limit throughout the three weeks and left him trailing one day in the Pyrenees.
Bodnar, taken in the straight line final of Pau (stage 11) after a long break, won for the first time in the Tour. I didn't think it would come down to this TT in Marseille.
"It was a good opportunity for us GC guys who have had a tough time in the Alps to regain a bit energy and sit on the wheels ahead of tomorrow's time trial".
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Aussie Michael Matthews was roared into the stadium in the green jersey and will look to end his magical Tour with a final-day win on the Champs Elysees.
Froome all but sealed his win on Saturday, finishing third in the time trial in Marseille and putting more time into Uran and Frenchman Romain Bardet, who dropped from second to third.
Team Sky have planned their tour to perfection, and once again, they have been the organisation with the most conclusive tactics.
Froome summed up in a word how he feels about sewing up his fourth Tour de France crown: "Incredible".
"I'm pleased to get through the Alps without any problems this year, because it's always the Alps that I find the hardest", he admitted.
"This morning, I didn't know what was going to happen".
A large breakaway group of some 54 riders had gradually splintered by the time the stage leaders reached the start of the Izoard climb. And one of those is a time trial race against the clock, a discipline he excels in - having provided him with bronze medals at the Olympics of 2012 and 2016.
"But I never dreamed of even coming close to Anquetil, Merckx, Hinault and Indurain and after this I have a new-found respect for them, for winning five". It's incredible. I have been here three hours waiting.
"But when you've got the yellow jersey in the team and what I believe to be the best rider in the world, you're in that position where you've got to defend yellow".