Reigning champions Peterborough were the more dominant side in the first half-hour of the game as they created more chances but were unable to convert them. The Red Dragons punished them in the 34th minute as Modou Faal found the back of the net with a clinical finish. The forward chased a long through ball from Max Cleworth and chipped the ball over goalkeeper Jed Steer from the edge of the box.
Four minutes later, George Dobson doubled the host's lead. Ryan Barnett's initial cross was intercepted by Jacob Mendy on the right side of the box and the left-back quickly squared the ball for Dobson to comfortably net the second goal. Despite a slow start, Phil Parkinson's side headed into the break with a two-goal cushion.
Peterborough looked determined to stage a comeback in the second period as just minutes after half-time they created an easy opportunity to pull one back, but the Posh had to wait until the 72nd minute to score as Wrexham goalkeeper Arthur Okonkwo brought down Cian Hayes inside the box. Malik Mothersille took the spot-kick and comfortably slotted the ball home. The visitors then scored a dramatic equaliser in the 91st minute as super-sub Bradley Ihionvien went past two Wrexham players before firing the ball into the net.
The game went straight to penalties and the visitors clinched a remarkable comeback as they won 4-2 in the shootout, with Paul Mullin missing his spot-kick. The Posh will feature in the final for the second time in a row on March 13 against Birmingham City at Wembley.