Madden had a great chance to score early on when Harry's Maguire's weak header back left goalkeeper George Long exposed but he fired wide of the target.
Madden put another effort off-target and Long was forced to turn a James Hayter shot over the bar.
United's best effort before the break came from Tony McMahon who sent a free-kick wide.
Madden opened the scoring in the 49th minute, creating space for himself inside the area before firing low past Long after receiving the ball from Byron Webster.
After McMahon put a long-range shot just over, Madden increased Yeovil's lead midway through the half following an error from Long.
The keeper hesitated on the ball and he was caught in possession by the alert Madden who was left with the simple task of walking the ball into the net.
Nick Blackman had a shot saved by Marek Stech as the hosts tried to pull a goal back but United have now suffered back-to-back home defeats.