Mexico striker Jimenez pounced on a miscued Tommy Doyle shot to finish with aplomb in the 90th minute at Bramall Lane.
Bidding to return to the Premier League after last season's relegation, Wolves climbed to seventh place in the table after their third win in six second-tier games.
United are in 14th place after their second defeat in seven league matches.
Wolves had a strong appeal for a penalty turned down when Mateus Mane tried to lift the ball over Mark McGuinness inside the area and it struck the defender's hand.
Jimenez, introduced off the bench in the 63rd minute, shot wide twice in quick succession, while fellow substitute Doyle saw his shot deflected off target.
But former Fulham star Jimenez provided the crucial contribution in the last minute of normal time.
Kieran Tripper found Doyle inside the area and his shot fell to Jimenez, who struck in predatory fashion at the far post.