The Norwegian, who has pledged to win every rally this year, drew first blood when he posted a superb time in the opening Saint-Disdier - Corps stage to go fastest by 10.1sec.
A steadier approach to the second and final stage of the day reduced his advantage to 8.9sec, but the Skoda Fabia Rally2 man was happy with his performance.
"It was a good start. On the second stage, we decided not to use the snow tyres we had in the back, but the conditions were very difficult. In hindsight, that may have been the wrong choice,” Mikkelsen explained.
“A good start, a clean start and the feeling in the car is very nice. Tyre choice tomorrow will be very crucial,” he added.
Adrien Fourmaux was second in an M-Sport Ford Fiesta Rally2. The Frenchman found a perfect rhythm during the second stage to take the fastest time whilst beating a handful of WRC runners in the process.
Volkswagen Polo pilot Nikolay Gryazin was 17.1sec further back in third, although the Russian driver has nominated Monte-Carlo as a non-scoring round.
Citroën C3 Rally2 star Eric Camilli was 12.7sec down on Gryazin in fourth, with Marco Bulacia ending the day fifth, 1min12.3sec adrift of the leaders in another Fabia.
Sean Johnston conquered his lack of Tarmac experience to end the leg 43.6sec behind Bulacia in sixth his Citroën, whilst Italian driver Enrico Brazzoli completed the leaderboard.His Skoda stopped for a handful of minutes during the first stage and he eventually finished 12min 04.3sec back from the top spot.