A win of any kind would have seen Lahti finish second in Group A, only for them to suffer a 4-0 defeat on the road to a side who were already eliminated.
Luke Pearce got the Aland Islanders of to a great start in the ninth minute, then Mariehamn chose the perfect time to double their lead, as Adam Larsson struck three minutes into first-half stoppage time.
The home side came out for the second half with all the momentum, and it took just three minutes for Pearce to add his second after the break, before Emmanuel Patut ended the game as a contest after just 55 minutes.
Reaching the last four would have been a real confidence booster for the newly-promoted side ahead of the league season beginning next month, but they had to settle for a fourth-placed finish in the group and seven points from five games, as Mariehamn overtook them.
That meant HJK, who won their final three games all by a 3-1 score line, held onto second place in the group, behind Inter Turku, who remain on course for a third successive Liiga Cup triumph.
With Group B also completed, the Semi-final line-up for the Finnish curtain-raiser is now set, with both matches scheduled for 17:00 CET on 14th March.