The Finn elected to scrub the tread from his tyres prior to the start in an attempt to improve grip and it was a decision that paid dividends as he set the pace around the street circuit in the area of the Catalonian city that once hosted the Spanish Grand Prix at Montjuic.
Volkswagen driver Mikkelsen recorded a time of three minutes and 39.1secs, finishing 1.8 seconds clear of team-mate and defending champion Sebastien Ogier.
"We've used some of tomorrow's allocation by scrubbing the tyres but we gained time," Mikkelsen said. "Maybe it will pay off or maybe not, but I'm happy with my decision."
Ogier missed the chance to clinch the title at the Rally de France as he was only able to claim three points due to gear-sensor problems on the second stage.
The Frenchman now needs to earn one point more than team-mate Jari-Matti Latvala, who is 27 points behind in the championship standings, to retain the crown he won in dominant fashion last year.
And, with Latvala only good enough for fourth behind Hyundai's Thierry Neuville, Ogier is in a good position heading into the second day, which sees the drivers tackle six stages in the penultimate rally of the season.