For City, it's a second consecutive season without silverware with their last Scottish Cup victory coming back in 2019.
Both sides were hugely frustrated to miss out on league glory but with City sealing second place and a Champions League spot, the hurt was perhaps more acute for Rangers who missed out entirely.
Head coach Jo Potter admitted how hard this week had been pre-match and it was clear early on that she, and her squad, were intent on putting things right.
McAulay was first to threaten when she took the ball on the run and fired across Lee Gibson to hit the base of the post.
The woodwork again rescued Glasgow City when Kathy Hill rose to loop a header on to the crossbar.
Leanne Ross' side were struggling to impact as they normally do but came close when Nicole Kozlova jinked and fired from outside the box, rattling the bar.
From there, Rangers took full control. McAulay escaped from Katie Wilkinson's through ball and her drive to the far corner was inch perfect.
A second soon arrived, when Wilkinson found Howat with her back to goal in the box. She led Van Diemen a merry dance before finding space and the left corner. An excellent finish.
A third seemed odds on when McAulay went clean through but Gibson somehow managed to block.
A half-time rethink was soon undone with Wilkinson once again the architect. She released Chelsea Cornet, who unselfishly squared for Howat to tap home from close range.
City struggled to find a route back and their day was summed up when Kozlova somehow placed wide when she had to score then Van Diemen walked for fouling substitute Camille Lafaix.