Plymouth took the lead after half an hour when a ball into the box was cleared as far as Conor Hourihane and he buried a fine shot inside the bottom-right corner.
Cheltenham had been on top before the opener with two excellent saves from Jake Cole denying former Argyle man Chris Zebroski.
Plymouth's lead only lasted for seven minutes as Steve Elliott leapt highest at the far post to nod in Billy Jones' free-kick. Hourihane went close to a second in first-half stoppage time but keeper Scott Brown made a fine block.
Plymouth appealed for a penalty in the third minute of the second half as Guy Madjo went down under a challenge from Elliott but referee Stephen Martin waved their claims away.
Mohamed then won it for Cheltenham, nodding in Jones' corner in the 78th minute before Plymouth's Alex MacDonald missed a glorious chance to level at the death as he nodded over with the goal gaping.