The Borderers gained revenge for a shock home defeat by the Seasiders in the Pembrokeshire KO Cup, and both teams and supporters deserved a medal for completing the fixture in the terrible wind and torrential rain.
Andrew Price, Molson Coors, GD Harries and FBM Holidays sponsored Tenby used home advantage and the benefit of the wind in the first half, and took the lead with a penalty by Jonathan Rogers after eight minutes, followed by another seven minutes later after the visitors lost their influential scrum-half Andrew Jenkins with a pulled hamstring.
Shortly after the break, Johnny Thomas fired over a penalty for the Borderers and outside-half Carwyn Jones dropped a goal with 19 minutes left, to level the score.
Few further scoring chances were created by either side, but a second penalty from Thomas in the final 10 minutes saw Whitland take the lead and the match.
"The match was played in terrible conditions, but there was some heroic defending from our players and we have now shown that we can compete with the higher division teams," said Tenby United first team manager, Peter James.
"We defended really well in a game that was never going to be open and expansive, which is a shame because it wasn't a true reflection of how far we've come.
"Every single player from both sides deserves credit for even playing the game in the driving wind and rain which caused both sides to keep it within the forwards.
"Division One Whitland's reaction to the win shows how well Tenby competed and showed how high a standard the boys are playing at, because our opponents were absolutely delighted and it was almost as if they had won the cup final! That is testament to our boys" hard work and dedication.
"We travel to Crymych tomorrow (Saturday) in the next round of the Pembs Cup after putting Whitland out of it earlier in the season, so that should be another good test for us and I'm sure the boys will do us proud.
"We'll be going up with huge respect for Crymych, but we will definitely be intent on getting a result, with the boys being confident that we can match the higher division teams, if not beat them," he added.
Tenby United: Damien Miller, Dai Meyrick, Toby Smith, Jack Guerreiro, Jack Price, Lloyd Thomas, Jonathan Rogers, Rob Clarke, Gareth Jones, Johnny Palmer, Chris James (capt), Luke Dedman, Roy Osborne, Luke Hansford, Patrick Roberts. Replacements - Greg Lees Griffiths; Wes Bonser; Jason Ronowitz; Rob Luly.
Whitland: Johnny Thomas, Matt Hayden, Scott Newton, Lee Glanville, Duncan Evans, Carwyn Jones, Andrew Jenkins, Aled Richards, Marc Jones (capt), Gethin Jones, Kieran White, Matthew Clarke, Jack Mason, Danny Evans, Callum Tree. Replacements - Zac France-Miller, Nathan Price, Will Davies, Nico Setaro.