Despite being outside the automatic promotion places at kick-off, Palace were arguably in the best position, playing bottom of the league knowing that a win would secure a top-two finish.
However, they were given an early shock as Portsmouth’s Megan Hornby scored direct from a corner in the third minute.
That seemed to jolt Palace into action and Molly-Mae Sharpe equalised from close range just eight minutes later.
Meanwhile, at The Valley, Charlton had begun to drop off after a bright start with Birmingham gradually gaining the upper hand.
Addicks’ goalkeeper Sophie Whitehouse was forced into action to keep out Oceane Hurtre’s well-struck effort.
The pressure eventually told in the 27th-minute as Leidhammar headed home Hurtre’s left-wing cross at the far post.
Birmingham remained in the ascendency with Leidhammar’s free-kick tipped onto the bar by Whitehouse before Batcheba Louis put the rebound over the bar.
And with Palace unable to make the most of their chances in the other game, that was how it stayed until the break – Birmingham top and Palace ahead of Charlton on goal difference.
It didn’t stay that way for long as Ashleigh Weerden rounded the goalkeeper to put Palace in front less than two minutes into the second half.
From there the floodgates opened for the Eagles with Weerden getting a second and Kirsty Howat making it four moments later.
Just as Palace’s promotion party was beginning at the VBS Community Stadium, Birmingham might just have been starting to put the champagne on ice themselves.
Charlton failed to deal with a corner from the right and Liedhammar was there to pounce, one touch before finding the top corner with a shot on the turn.
Palace made it six against Portsmouth through Annabel Blanchard and Elise Hughes, and with Charlton never looking like scoring, the only question was whether Jo Potter’s side could keep scoring to pip Birmingham to the title on goal difference – 11-1 would have done it.
As it was, they had to settle for six and, although a Charlton flurry finally came in the 13 minutes of added time, Birmingham held on comfortably enough with the final whistle sparking wild celebrations.