The 20-year-old has signed a season-long loan deal with English League One side Sheffield Wednesday.
Hearts were keen to bring the versatile attacker to Tynecastle Park in a bid to bolster Robbie Neilson’s attacking options.
It was decided a move to the third tier of England where he will likely be involved in a promotion battle was preferable.
A quick forward, who can play on either flank, Mighten has 66 appearances for Forest and came off the bench in their first Premier League encounter.
"Really please, it's a massive club,” he said on signing for Wednesday. “It's a big responsibility every time you get on the pitch so I am just happy I’m able to contribute hopefully this season.
"I’ve had several conversations with the manager [Darren Moore]. He's a key factor as to why I am here today. I believe in his way of playing, his style. Obviously the size of the club is a big factor as well. I’m excited and I hope we can get back up to the Championship.”
With reports that Liam Boyce could be out for the remainder of the season, there is a pressing need for Hearts to add attacking options before the transfer window closes on Thursday, especially with the team entering a period of playing twice a week.