The American-Icelandic striker struck right before halftime to help his side to a win in the first Eredivisie match following the winter break.
Steven Berghuis had given AZ the lead before Johannsson got on the end of a cross in first-half stoppage time and bundled home with his chest to make it 2-0. Berghuis added a second goal in the second half to secure the win.
The goal was the 12th league strike of the season for Johannsson, and his 19th in all competitions.
With the win, AZ moved up to seventh place in the Eredivisie table.