The 29-30 win means the Drovers top Pool 8, three points clear of the Knights in second place.
After Doncaster went home from Church Bank last week with the win, few gave Llandovery a chance of reversing that defeat but they came through with a last minute try from their diminutive centre Jack Roberts.
The Drovers also gained a valuable bonus point for scoring four tries and top Pool 8 with three wins from four games as a result. The Premiership side now host Melrose and travel to Nottingham in the New Year and remain favourites to make the knockout stages.
Despite the big Doncaster pack providing a physical threat, the Drovers got more ball than in the game at Church Bank but trailed 22-18 at the break.
Two tries from scrum half Chris Hallam and a penalty try had bettered tries from centre Adam Warren and hooker Luke Lewis.
Flanker Lloyd Phillips and Doncaster hooker Rhys Buckley swapped tries but in a grandstand finish, Roberts tore through for the winner.
