But the 20-year-old on-loan Arsenal defender redeemed himself by netting from a corner to secure Kilmarnock a 1-1 draw that moves them a point above Scottish Premiership bottom club Hamilton.
Medley's first career goal, and his team's first in six matches, was reward for some sustained pressure. But Killie needed three impressive late saves from Colin Doyle to save them from a ninth consecutive defeat.
The point was Tommy Wright's first as Killie manager following 1-0 defeats in his opening three games while United saw their top-six hopes take a major blow.
Micky Mellon's side fell four points adrift of sixth-placed St Mirren with only games against Celtic and Aberdeen to come before the split.
Medley showed real character to bounce back after a shaky start. United had a chance in the first minute when they got Lawrence Shankland in behind after the centre-back had committed himself. The striker fired just wide of the near post.
Killie came close to breaking their barren spell just before the opener when Marc McNulty was loose with a pass back towards his own box. Kyle Lafferty curled a shot which dipped on to the bar on his first start for Killie.
The opening goal came in the 18th minute after Ian Harkes dispossessed Medley and fed Sporle, whose deflected shot bounced into the far corner.
Sporle only lasted another 10 minutes before going off injured. Louis Appere took his place.
Alan Power then swept a shot over the bar after a good move.
Killie remained on top at the start of the second half and Medley produced a powerful volley that was well saved by Siegrist.
Mellon responded with a tactical switch by putting on central defender Mark Connolly for midfielder Jeando Fuchs.
There were some frantic moments in the United box before the 64th-minute equaliser. Lafferty flicked on Chris Burke's corner and Medley delivered a cool finish at the far post.
Burke and fellow substitute Nicke Kabamba were off target before Connolly was booked for pulling down the striker as he chased a ball in behind. The fact Kabamba was running towards the corner saved the former Killie player from the red card the home players were calling for.
The hosts had the ball in the net from the resulting free-kick but Munro blew for a foul on Siegrist.
Killie continued to press but were almost caught out after their own corner when Shankland brilliantly brought down Liam Smith's long diagonal ball.
Colin Doyle saved from the striker and produced two more impressive stops from McNulty and Shankland as United ended the game strongly.