The Australian took the yellow jersey on Saturday after finishing second on stage seven, reaching the La Plagne summit behind Mark Padun.
Porte made his attack on the mountain stage after being given the all-clear from Ineos Grenadiers teammate Geraint Thomas.
It was a big day for Australian riders with Jack Haig finishing the day nine seconds behind in fourth, while Ben O'Connor finished fifth a further four seconds back.
Porte followed race winner Padun and Enric Mas of Movistar in an attack on the final climb dropping Sepp Kuss of Jumbo-Visma in the process.
The 36-year-old Tasmanian showed he still has plenty left in the tank by out duelling Mas and powering in the final kilometre to take second and the yellow jersey.
"At the end of the day, it was a good move. It's nice to have numbers and a strong team to be able to do that," Porte said.
"When Sepp Kuss and Padun went, I was more worried about Mas, who wouldn't work with me, but I guess that's down to the tactics. It worked perfectly as Geraint and Tao [Geoghegan Hart] were back there."
Porte sits 17 seconds in front of Kazakhstan's Alexey Lutsenko and 29 ahead of Welsh star Thomas, with the final stage on Sunday.
Haig moves up to fifth in the GC standings 34 seconds behind Porte, while O'Connor goes into the final stage in eighth one minute in arrears.
