Mignot scored once in each half - both at the back of rolling mauls - and Les Bleues held off a late rally from England to seal a 17-12 victory and their second title in the last four years.
For the Red Rose - who took the lead in the first half thanks to Lydia Thompson's first-half score - it was a case of what might have been as they came within a whisker of ending their four-year wait for a title.
After last year's disappointing fourth-place finish there is plenty of cause for optimism for Scott Bemand's young side but they were often powerless against a dominant French forward display.
France lost their chance of a Grand Slam with defeat to Wales last month but with a points difference 21 points stronger than England, they knew that victory at the Stade de la Rabine would give them the crown.
However it was the visitors who started on top in Brittany but could not get over the whitewash despite dominating territory and possession in the first quarter.
But just when it looked like they might have wasted their chance, Thompson scored a superb solo effort - breaking three tackles on her way to the line - Amber Reed adding the extras.
The French forwards had not yet arrived at the party but after the half-hour mark they began to assert themselves.
Their first rolling maul effort was denied by the TMO but moments later they had their first score and it was Mignot who emerged with the ball.
Camille Cabalou missed with the conversion however and at the break the Grand Slam was still on, the visitors leading 7-5.
It looked like it might just be England's night when Cabalou hit the post with a penalty soon after the interval but the French pack soon got back to work - this time it was Audrey Forlani who profited from an unstoppable lineout maul. Cabalou made the extras count this time and France were in front at 12-7.
But not for nothing are England the World Cup champions and this young side summoned an immediate response.
A superb driving maul went over the line and while it was hard to know which English player had actually touched down - the try was awarded nonetheless and it was later revealed to be Tamara Taylor who had claimed it.
Reed's conversion hit the post and the scores were level at 12-12 - at this rate England were winning the Women's Six Nations but not the Grand Slam.
However after the hour mark the French once again turned to their power up front and it was skipper Mignot who was again the beneficiary at the back of a rolling maul.
Cabalou missed the extras and at 17-12 England still had a chance. They managed to assert some pressure in the French 22 near the end but the blue defensive wall held firm and France had broken English hearts to take their second crown in four years.
Click here for 2016 Women's Six Nations table
– but the sailors praise the new boats