Bonnyrigg scored two first-half goals against the 10 men of Cowdenbeath to help take a commanding 3-0 lead into the second leg of their League Two play-off final.
The match swung against the visitors just after the half-hour mark when Harvey Swann picked up a second yellow card for lunging in on Dean Brett.
It did not take long for the non-league hosts to make use of the man advantage when Sean Brown bundled over from close range.
Bonnyrigg then doubled their lead just before the break courtesy of Neil Martyniuk from the penalty spot after Callum Connolly was brought down inside the area.
And the hosts ensured they would take a healthy advantage into the return leg when Dean Brett was left unmarked at the back post before nodding home to make it three.