Cassidy, on loan from Wolves, grabbed his 10th goal of the season with just five minutes remaining on the clock to cancel out Paul Huntington’s headed opener and ensure the spoils were shared.
After a slow start from both sides the match sprung into life after 35 minutes when Tranmere goalkeeper Owain Fon Williams was forced to turn away Stuart Beavon’s powerful effort from distance.
Cassidy, 19, fired wide from close range for the home side before Beavon again went close, this time seeing his header cleared off the line by Ash Taylor on the stroke of half-time.
But Huntington broke the deadlock after 50 minutes, heading into an empty net after Jeffrey Monakana’s cross came back off the crossbar.
Soon after Jack King should have doubled the Lilywhites’ lead when clean through on goal but he fluffed his lines, before Cassidy slotted home from a tight angle after James Wallace had sent him clear to claim a point for Rovers.