Despite beginning the round in fourth place, HJK knew they could get right back into the title race and move to within two points of leaders Inter Turku, who were beaten for the second time in as many games on Saturday.
The record Finnish champions grabbed the chance with both hands and went ahead after only seven minutes through Vicente Besuijen, but the floodgates really opened soon after when Alex Ring was hauled down in the area by Pontus Lindgren, who was shown a straight red card.
Ring stepped up and scored the penalty, before HJK were awarded a second penalty with just under 30 minutes on the clock, which Ring put low to the goalkeeper's left, a carbon copy of his first spot-kick.
Korede Adedoyin gave Mariehamn a glimmer of hope when he pulled a goal back before the break, but that hope was dashed in the opening seconds of the second half when Santeri Hostikka set up Pyry Mentu for HJK's fourth.
Ring completed his hat-trick shortly after the hour mark to make it 5-1, and though Niilo Kujasalo pulled another goal back, HJK failed to relent - Brooklyn Lyons-Foster and Hostikka added two more in quick succession, before Lyons-Foster made it 8-2 in the 89th minute with the goal of the game.
Meeting Lucas Lingman's corner in front of the near post, he produced an audacious flick over the goalkeeper and into the far corner to cap off a memorable win in real style.
The 8-2 defeat officially ends Mariehamn's chances of reaching the Championship Round, a week after they finally ended a seven-match winless run.
Despite eight goals and closing in on first place, HJK stay fourth in the table thanks to KuPS, who beat Jaro 3-1 away from home to move level on points with FC Inter and end the newly-promoted side's dream of reaching the Championship Round.
Things began brilliantly for the home side who took the lead midway through the first half as goal-machine Kerfala Cissoko turned provider on this occasion for Rudi Vikstrom to score, but six minutes before the break, a first league goal of the season for Saku Savolainen drew KuPS level.
The match was won and lost in a six-minute spell midway through the second half - a powerful Petteri Pennanen strike off the post put the Kuopio side in front, before they doubled their lead from the penalty spot through substitute Agon Sadiku.
The final nail in the Jaro coffin came on Monday night, as Gnistan won 3-2 at home to AC Oulu to move back into sixth place, three points clear of VPS with one round of matches to go before the league splits into two, while Jaro stay a further three behind the Vaasa club.
Gnistan were the favourites against the side sitting in 11th, and made a positive start when Tim Vayrynen scored from the penalty spot after seven minutes, but were pegged back midway through the half when Niklas Jokelainen also notched from the penalty spot.
Evgeniy Bashkirov put the home side back in front with 16 minutes to go, before a fourth goal in as many games for Rachide Gnanou gave Gnistan some breathing space eight minutes from time, before Julius Paananen added a consolation for Oulu.
Having knocked Jaro out of the race, Gnistan need just a point against the same side on Sunday to finish sixth, but should they lose, a win by any score against Inter Turku for VPS will see them snatch the last Championship Round spot.
Monday's other match went with the form book, but only narrowly, as SJK scored a late winner to beat Haka 3-2 away from home.
Two Kalle Multanen penalties in the second half came in between Kasper Paananen's early goal for the visitors and Jeremiah Streng's equaliser with a quarter of the game to go, before Kevin Appiah Nyarko won it for SJK in the 91st minute, just two minutes after coming on.
It's a fourth win in a row for SJK who stay fifth but are now just three points off the top, while Haka have now lost five on the spin and have taken one point from their last eight matches.