Extra time brought with it very few clear chances for Albion to swing the pendulum in their favour.
Mohammed, who could’ve poked a winner home in normal time, headed a couple of crosses over, while Cardiff stood resolute as they looked for a goal of their own.
In the end, neither could be separated - meaning a penalty shootout was required to decide the victor.
Each team traded four successful penalties before Cody Twose’s effort went just wide of the treat for the Bluebirds, allowing Onyemschi to convert the winning spot-kick and send the Baggies through to the last eight of the PL Cup.
Albion: Wallis, Nelson, Diomande (Kirton 56), Mohammed, Parker, Hall (c), Mandey (Iddrisa 46), Whitwell, Williams, Humphries (Ranger 46), Higgins (Onyemachi 93).
Substitutes: Cisse.