Casey played the difficult Spyglass Hill on Saturday but separated himself from the field with a six-birdie, one-bogey round to get to 15-under for the event. Despite his lead, the tournament is far from over as Casey has failed to convert his previous two 54-hole leads of three shots or more, according to the Associated Press's Doug Ferguson.
Mickelson - a four-time Pebble Beach Pro-Am champion - leads the chase of Casey. He'll be looking for his 17th PGA Tour win when trailing after Round 3 to add to his career total of 43. Lefty made cut this week, marking the 500th time he's done so on the PGA Tour.
It was a disappointing back nine for Jordan Spieth, who played his final six holes at 4-over par. He fell into a tie for 18th after jockeying for position near the top of the leaderboard for most of the day.