The Icelandic-American scored twice in the first half as his side easily saw off Heracles, which was reduced to 10 men with a 24th-minute red card.
The 23-year-old U.S. national team striker opened the scoring in the 16th minute, latching onto a cross and taking his first touch past his defender before volleying home from close range with his second touch.
Johannsson was at it again with a fantastic bit of skill in the box 10 minutes before the break. The striker received the ball at the 6-yard box with his back to goal before executing a 360-degree spin past two defenders, feigning the goalkeeper and finishing into an empty net.
Mattias Johansson would round out the scoring on a great afternoon for the home side.
The goals were Johannsson's 15th and 16th in league play this season, and 23rd and 24th across all competitions. The striker is chasing Jozy Altidore's American abroad record of 31 goals all competitions, set last year with AZ.