Johnson signed with MLS on February 17, 2012 and was selected on February 18, 2012 by Montreal Impact via allocation process. He was immediately traded to Seattle Sounders FC in exchange for Mike Fucito and Lamar Neagle. He scored his first two goals for the Sounders in games against the Chicago Fire and the LA Galaxy. He was selected to the 2012 MLS All Star Team by coach Ben Olsen, eventually scoring the game-winning goal against Chelsea in a 3-2 victory. During the 2012 season, Johnson was selected as MLS Player of the Week two times. The first time in week 22, following a one-goal performance in a 4-0 win over the Los Angeles Galaxy. He received the honor a second time in week 27, following his two-goal showing in a 2-1 comeback victory against Chivas USA.