The full-back, currently on loan with the Robins, netted the decisive spot-kick in their penalty shoot-out victory over Charlton on Monday to set up a Wembley date with Millwall.
McMahon told liverpoolfc.tv: "He was terrific. If ever the game was set up for someone, it was Darbs.
"His temperament is fantastic and he was coolness personified with the penalty. He was calm and assured - it was a great penalty for a full-back."
McMahon believes Darby's composure in the drama of the shoot-out is testament to the defender's mental toughness.
"He is unbelievably strong," he said. "Physically he doesn't look the strongest, but he is. The mental side of Darbs is excellent too - he is a tough character.
"He will work hard, and when things are getting tough he has the mental strength to find more from somewhere else."
McMahon added: "Going out on loan has done him the power of good and we've had some excellent feedback from Swindon manager Danny Wilson about Darbs.
"He's told us what a good lad he thinks Darbs is off the pitch and in the dressing room, and also what a credit he is.
"I am so pleased for him and I just hope he can go on and win the final now."