Dempsey had played two road games with Seattle. An announced crowd of 67,385 showed up for his home debut -- the second-largest stand-alone crowd in league history.
Johnson scored in the 60th minute on a header to send the Sounders to their fourth loss of the MLS season.
Seattle, with 37 points, is tied with FC Dallas for fifth place in the Western Conference.
Portland, with 39 , is tied with Colorado for third place in the conference.