The Barking-born midfielder, 24, hit their second goal, then the winner after bitter rivals Cardiff had levelled from 2-0 down.
Boss Paolo Sousa said: "Darren had a great game today. He showed everybody what he can do.
"There are still lots of things he needs to work on but he's a player with lots of potential."
Nathan Dyer headed Swansea in front after nine minutes and Pratley doubled the lead in the 16th minute when he smashed home Craig Beattie's knockdown.
But Cardiff hit back before the break with two goals in three minutes.
Jay Bothroyd beat Dorus De Vries with a cool finish and Mark Hudson headed home Ross McCormack's free-kick.
But, in the 61st minute, Pratley volleyed home from Fede Bessone's cross.
Cardiff boss Dave Jones said: "We gave away crazy goals. We got back to 2-2 but weren't playing well."
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