The Australian midfielder has been earning rave reviews at the heart of the Millwall midfield this season and stole all the headlines once again to hand Dennis Wise his first victory as player-manager in the FA Cup.
Despite ending the game with nine men, it was Walsall who opened the scoring after just 12 minutes when Darren Wrack's harmless looking cross caused all kinds of problems for Millwall defender David Livermore.
Livermore made a crucial mistake when he attempted to chest the ball back to keeper Tony Warner and Jorge Leitao capitalised to fire the ball past the Millwall keeper.
The home side got their just rewards when they drew level on 33 minutes. Neil Harris weaved his way into the penalty box before laying the ball across to his strike partner Kevin Braniff.
Braniff made no mistake sliding the ball past the stranded Walsall keeper James Walker for his first ever FA Cup goal.
The goal seemed to ignite Millwall and they took the lead on the stroke of half-time. Wise's deep cross into the penalty box fell perfectly for Cahill who planted a header past the stranded Walker into the corner of the net.
Walsall's hopes of earning a replay evaporated on 69 minutes when they were reduced to ten men after Leitao was shown his second yellow card of the game for a rash challenge on Wise.
The game descended into a farce in injury time when Walsall keeper Walker was handed a straight red card for pushing Wise in the chest.