Last week the 23-year-old Egan had his association ended with owner Prince Faisal, who has reverted to using the "best available jockey".
Trained by John and Thady Gosden, Mishriff went on to win the Juddmonte International by six lengths last season and showed he retained all his old spark when finishing fast for second in the Eclipse recently.
Doyle's agent confirmed on Tuesday morning that the 34-year-old will take the ride on Mishriff in the King George.
The rider has enjoyed a stellar season already, winning both the 1000 and 2000 Guineas on Cachet and Coroebus respectively and is delighted to team up with Mishriff.
He told Nick Luck's Daily Podcast: "I couldn't be more thankful to the connections for putting their trust in me and choosing me to ride him.
"He's an incredible horse and what he has achieved is just outstanding. I thought his run in the Eclipse showed he is still right at the top of his game.
"I'm going into Mr Gosden's tomorrow morning to pop on him as obviously I've not ridden him before, but I've seen a fair bit of him and it will be nice to jump on board before Saturday.
"He travels into his races really well and I thought he was a shade unlucky in the Eclipse. it was an incredible run really to get stopped the way he did and when he got out he really hit the line hard."