Only winless Mariehamn were below SJK at kick-off, with the hosts losing their previous three in a row, while HJK arrived looking to go third with a third win in a row.
The first half set the tone for a frustrating evening for the away side, who created decent opportunities from the majority of possession, but couldn't find a breakthrough.
HJK were punished for that with 10 minutes played in the second half - SJK moved the ball directly down their left flank, before Eetu Mommo's cross was turned in by Kelwin. Five minutes later, the hosts struck again, as Eze Onuoha found Jeremiah Streng, who grabbed his fourth of the season.
Incredibly, it took SJK another five minutes to add a third and stun the Helsinki side. Murilo Henrique won the ball on the left-hand touchline at halfway, then kept running and running, and just when he looked set to pull the ball back, he caught the goalkeeper out at the near post to score from a very tight angle.
Just a fourth win of the league season takes SJK above Jaro into 10th and ease some relegation fears, while HJK are just three points clear of the relegation group places with four regular season matches to play.
Follow the climax to the regular Veikkausliiga season on Flashscore.