As revealed by skysports.com West Brom launched a bid for Mattock as they look to bring the highly-rated defender to the Hawthorns.
The two clubs were struggling to agree a fee for Mattock forcing the full-back to hand in a transfer request in a bid to secure a move.
However, skysports.com understands a deal has now struck between both clubs to see Mattock make the switch to West Brom.
Mattock has now been given permission to hold talks and undergo a medical at West Brom.
Foxes boss Nigel Pearson confirmed he expects Mattock to seal the move soon.
"It is a situation that has been driven by him," Pearson said of the transfer request.
"In terms of confirming where he goes and for how much, I can't do that, but I'd expect it to be resolved shortly."