The Australian, who lives in Monaco and trains on the 9.6km climb regularly, set a time of 19 minutes 16 seconds.
That was a huge 23s quicker than Garmin-Sharp’s Andrew Talanksy, whom he led by 32s ahead of the stage and finished second on the day. The American therefore finishes second on GC, 55s seconds behind Porte, and triumphs in the young rider classification.
AG2R La Mondiale's Jean-Christophe Peraud (+32s) completes the podium, 1:21 off the winning time, after finishing fourth behind Colombian climber Nairo Alexander Quintana Rojas of Movistar (+27s) on the stage.
American Tejay van Garderen (BMC) is fourth overall after taking fifth spot, while Saturday’s stage winner Sylvain Chavanel (Omega Pharma-Quick Step) was ninth to end in fifth on GC.
Chavanel won the points leader's green jersey while Johann Tschopp had already won the King of the Mountains polka-dot jersey prior to the stage.
Dutchman Lieuwe Westra (Vacansoleil-DCM) - like Chavanel only 42s down on Porte ahead of the final stage - never threatened. He finishes eighth.
Last year Porte was down in 28th in the time trial, 1:44 down on team-mate Bradley Wiggins’s winning time of 19:12 - but that was largely a result of it coming at the end of a brutal week of riding as a domestique for the British winner of the race and Tour de France.
"I can't believe it," said Porte. "I'm the first Aussie to win [this race] and it's just an honour to have my name up there with all the big champions like Wiggins."
Wiggins did not ride in Paris-Nice this year as he is focusing on the Giro d’Italia.
The stage began at the base of a climb which boasts an average gradient of 4.7%, with some slopes as steep as 8.6%.
Porte has said he will revert to a supporting role for Wiggins and Chris Froome - tipped to be Sky’s focus on this year’s Tour and the new leader of the Tirreno-Adriatico with two stages to race – after Paris-Nice.
"I'm still doing my apprenticeship and learning off Brad and Chris and I don't expect to ride for general classification at the Tour - maybe next year at the Giro," said Porte.
"I'm in a good place at the moment and don't really want to change anything. Whatever happens we're going to have a hell of a team for the Tour de France."
