On the second playoff hole against Troy Matteson, Johnson hit from the fairway bunker on the left side of the 18th hole at TPC Deere Run. The first time, his ball landed left in the water hazard. The second time, his ball stopped rolling a foot from the cup.
The birdie was enough to beat Matteson and capture Johnson's second PGA Tour title of the season. Johnson, a native of Iowa and a local favorite in this event, won at Colonial in May.
Matteson made a 60-foot eagle putt at the par-5 17th to tie Johnson at 20 under. Both made par at the finishing hole to force the playoff, Matteson shooting 2-under 69 while Johnson carded 65.
Steve Stricker's reign as three-time defending champion ended on Sunday as he closed with 1-under 70 to finish four strokes out of the playoff and tied for fifth place with rookie Luke Guthrie.
Scott Brown finished alone in seventh at 15 under par.
As the highest finishing player in the field not already exempt for the British Open, Matteson is in the field at Royal Lytham & St. Annes.