The Red Roses saw a 31-7 lead after 23 minutes vanish on Saturday before a thrilling French fightback that eventually ran out of time, with Joanna Grisez's marauding try with two minutes left their final score.
England are favourites to win their home World Cup but their descent from being in complete control to scrambling to save the game will have given their title rivals encouragement.
"This is ideal for us. After a really good start we could have put them away but they got energy through the middle and that's what French sides do. Once they get into you, they feel success," said former All Blacks coach Mitchell.
"I thought we'd have to win the game three or four times but I think we had to win it nine times. We'd done enough to complete the goal.
"These types of games do help us. We get them occasionally. But you also have to understand that the Red Roses are unique at the moment and have been for a while. Teams tend to perform against us.
"That will continue to happen in 2025 and that will help us raise our game. And we have to raise our game, which is really clear."
Mitchell's chief concern was the alarming defensive collapse that saw France run amok out wide with England players falling off tackle after tackle.
"I wasn't happy with the defence. They got through us in the middle too easily. We adjusted at half-time and the girls did a good job in that area in the second half. But the edge defence was not the standard of the Red Roses," the head coach said.
England, who extended their record winning streak in the tournament to 34 matches, were clinical during a breathless first half with tries from Abby Dow, Emma Sing (2), Lark Atkin-Davies and Claudia MacDonald, but they leaked three scores to the visitors, squandering a 31-7 lead and go in 31-21 up at halftime.
The game sustained its breakneck, tit-for-tat pace in the second half as England captain Zoe Aldcroft finished after Dow cut through before French winger Kelly Arbey replied.
France were never out of it and when Grisez scorched over down the left to reduce the deficit to just one point with a minute left, the upset for an England team that had not lost a Six Nations game since 2018 looked on.
It was a showcase of attacking rugby from both sides that left serious questions about England's defence ahead of the global showpiece they are desperate to win while exorcising the demons of their defeat in the 2021 final to New Zealand.
