Arensman crossed the finish line in the rain two seconds ahead of title contender Jonas Vingegaard and his great rival Pogacar.
However barring a collapse Team UAE rider Pogacar should claim his fourth Tour de France title aged just 26 years in Paris on Sunday.
The Slovenian exits the Alps with a lead of four minutes and twenty-four seconds on second placed Vingegaard with just two stages remaining.
German breakout star Florian Lipowitz strengthened his hold on third place and top spot in the under-25’s section on his debut Tour de France as he tracked the title rivals all the way to the summit finish.
For Arensman and Ineos this was a symbolic victory as the British team begin their revival with the return to the helm of Dave Brailsford.
Arensman slumped to the ground in tears at La Plagne as he just held on with a white-knuckle struggle over the final kilometres.
He previously won stage 14 at another ski resort, at altitude in the Pyrenees at Superbagneres.
This final mountain stage was shortened overnight due to an outbreak of a cattle infection along the route, reduced to just 93km, which ran to a rowdy finish at 2050m altitude.
The stage however was largely uncontested in comparison to Thursday’s epic cat-and-mouse between Pogacar and Vingegaard, who attacked the champion 71km from the finish line without ever dislodging him.
Stage 20 on Saturday is a hilly run through the Jura, while Sunday’s run to the Champs Elysees also features a potentially chaotic run up the cobbled lanes of Montmartre. (AFP)