The world number one scored his second consecutive PGA Tour win with a near-flawless final round in Indiana to underline his form going into this month's Ryder Cup.
Rory McIlroy sends Ryder Cup warning with BMW Championship win
USPGA champion McIlroy sank six birdies while a bogey on the final hole was the only blemish on his card as he climbed from an overnight position of joint third to claim the title on 20 under.
"I came here with the mindset from Boston that I just wanted to keep going on this roll," said the 23-year-old, whose previous triumph lifted him to number one in the FedEx Cup standings and ensured he will have a shot at the $10million (Dh37.6m)prize at the Tour Championship in Atlanta later this month.
"Some suggested that I could have taken a week off and still could have been in the top five in the FedEx Cup standings going into Atlanta, but I felt like I was playing really well and didn't want to stop that run.
"I sort of picked up where I left off in Boston, shot 64 the first day here, and just playing with a lot of confidence right now.
"I'm confident in my ability and confident with the shots that I'm hitting and confident on the greens. I'm making the right decisions out there, and everything is really just going to plan at the minute.
"It's a nice run to be on, and I want to try to keep it going for as long as possible."
Mickelson, who had held a share of the lead with Vijay Singh after a stunning round of 64 on Saturday, carded a final-round 70 and had to settle for joint second alongside Westwood (69).
American pair Tiger Woods (68) and Robert Garrigus (69) were a further stroke adrift on a congested leaderboard.
Woods was four under through his final 11 holes but could not get himself into contention after a slow start.
"As I said yesterday, it was going to take about a 63 or 64 today to give myself a chance, and that was the number," the 36-year-old 14-time major winner said.
"With the pins, we saw the dots for today ... and it could be done. Because the greens were so soft, and if you take care of the par fives, throw in a couple more ... all of a sudden you have your number."