Mickelson has added the event to his schedule and been given the final sponsor's invitation after a run in which he withdrew from The Memorial, finished only 65th at the US Open and then missed the cut - along with Tiger Woods - at the Greenbrier Classic on Friday night.
It was the first time Mickelson and Woods had crashed out early from the same event in their professional careers.
He came only 58th at Castle Stuart last year in an event reduced to 54 holes because of bad weather, but a week later was joint runner-up to Darren Clarke in The Open at Sandwich.
That was the 42-year-old's best-ever finish at the championship, although he ended up three strokes behind Clarke, whereas at Troon in 2004 he was only one behind play-off pair Todd Hamilton and Ernie Els.
The left-hander's missed cut in West Virginia took his number of consecutive rounds without breaking par to seven, a career worst.
"I really have been off," he said. "I don't know what to say about that.
"The parts don't feel that far off, but I haven't been putting them together.
"It doesn't feel bad off the tee, it doesn't feel bad with the iron play, it doesn't feel bad chipping or putting, but I'm making a lot of loose drives here or there, some loose iron shots here or there, missing some short putts here or there."