Cronin, from Ireland, took the lead on day two after his chief rival Alastair Fisher crashed out. He held the position until the end of the rally, taking eight stages wins along the way.
Cronin is now second in the WRC 3 driver standings, 20 points behind championship leader Chardonnet.
“This is the first time we have competed at Rally Finland but we really had a good feeling on these roads which are still very special,” said a delighted Cronin, who now heads Citroen’s own Top Driver Series.
“The DS3 R3 was perfect and fast and we can’t wait for Germany to begin the series of tarmac rallies. We’re going to give it our best shot to hold onto our position as leader of Citroen’s series.”
Chardonnet had a tough opening day but fought back on Friday, overhauling local driver Vainionpää to establish a clear lead in second spot. He held the position throughout Saturday’s stages to finish 1m14.5s adrift of Cronin.
With his championship lead intact, the Côte d’Azur-based driver admitted he was pleased with his weekend’s work.
“It’s true that the start of the race was complicated for us, but we then found a good feeling in the car and the DS3 R3 was really great,” he said.
WRC 3 newcomer Vainionpää enjoyed an intelligent race. He drove consistently, taking no unnecessary risks to finish 2m43.1s behind Cronin on his first rally in a Citroën DS3 R3.
Simone Campedelli finished just outside the top three. After a testing start to the season, the Italian driver enjoyed a controlled race, also winning two stages and fighting for third until the last few kilometres.
Frenchman Quentin Gilbert was less fortunate. Hampered by technical problems and missing a corner on the first run on Ouninpohja, he finished fourth.
Stéphane Consani was the last of the WRC 3 finishers, and admitted afterwards that it was a weekend that he would rather forget. After suffering a double puncture on Himos 2, he went off on the eleventh stage and had to rejoin under Rally2 rules, eventually finishing 44 minutes behind Cronin.