Mabil smashed his penalty hard and low to his left-side as the Bronby goalkeeper went the other way.
The victory is Midtjylland's first Danish Cup after being beaten in the final four times.
Mabil started on the bench and came on in the 106th minute with the score at 1-1.
The 23-year-old has played 40 times this season for Midtjylland, scoring six goals and producing 11 assists.