'This is in no way a reflection on the tournament, which is one of my favorites on tour,' he said.
Mickelson cited personal reasons, although he hinted over the weekend at the Wachovia Championship that he was tired and needed a break before playing his way to the U.S. Open. He took only two weeks off after winning the Masters, wanting to play in New Orleans.
'I think I might just need a little break and try to get refreshed before the U.S. Open,' Mickelson said from Quail Hollow.
He plans to play the Memorial and Barclays Classic before going to Winged Foot for the U.S. Open, where he will try to win his third consecutive major.
Tiger Woods had planned to play in the Byron Nelson Championship until his ailing father took a turn for the worse. Earl Woods died Wednesday, and the funeral was Friday. Woods has not said when he will return.