Facing their old Pembrokeshire League rivals in the closest thing the United can get to a derby encounter in National League division three (west) this season, Tenby failed to capitalise on better first half pressure and paid the price after the break, going down by 22 points to 15.
The reverse was a blow to the resurgent United's fortunes and, while they still hold a three-point, game-in-hand cushion over bottom-placed Felinfoel - who they meet on December 1 - they must be kicking themselves for not getting more points on the board in the opening 40 minutes when they had Cardigan on the rack for long periods.
After a scrappy opening, Tenby - although missing Mike Evans - started to assert control in the set-pieces, with scrum-half Neil Duffy providing sterling service around the fringes to close down Cardigan counterpart Eurig Lloyd and prevent him from utilising any opposition ball won.
Although the United pack refused to let the Cardigan eight intimidate them, outmuscling them in the loose to win turn-over ball, outside-half Neil Powling was unable to exploit gaps behind the visitors' backline which allowed their pacy threequarter line to counter.
Aggressive defence from the Seasiders kept Cardigan at bay and the way the United took their opponents on up front made many of the Tenby faithful wonder if both sides could maintain such a frenetic pace for a full 80 minutes!
Twenty minutes in, Cardigan lost the services of stand-off Aled Thomas, with his replacement by Alun Williams necessitating a reshuffle in the Cardigan threes which brought centre Chris Liptrot over to pivot.
The United continued to put Cardigan under pressure, with Duffy forging his way through from a ruck.
Straight running centre Paul Luger drove deeper into the heart of the visiting defence from second phase and the Seasiders recycled again, but their efforts to gain fourth phase ball were thwarted by a little tardiness at the breakdown.
However, an infringement by Cardigan netted the United a penalty, which Powling slotted between the sticks to open the scoring.
Tenby's lead was shortlived, however, with captain Barry John and fullback Gareth Davies showing their pace from a quick tap penalty to wrong-foot the United defence and send wing John Mathias over for an unconverted try.
Although Tenby enjoyed a territorial and possessional advantage, they were unable to turn the pressure into points, with Cardigan providing blanket cover in their 22.
Another Cardigan breakaway netted them a scrum five metres out, with number eight Andrew Phillips breaking blind off the back to send scrum-half Lloyd over for a touchdown.
Liptrot converted to put the visitors nine points in front against the run of play.
Tenby countered from the restart, with Cardi lock Darren Morgan being shown the yellow card for obstruction when Powling floated a chip over the line for wing Nicky Rees to chase.
Powling buried the resultant penalty into touch on the corner flag, with Paul Broaders taking possession at the lineout to set up a catch and drive.
The Seasiders controlled the ball over the line for prop Lee Tucker to claim the touchdown.
Powling landed the conversion from wide out to reduce the deficit to two points at the break.
Coming out after the change around, Cardigan were still without Morgan, who had some eight minutes left to serve in the sin bin, but Tenby failed to make their numerical advantage up front count.
In fact, they fell further behind just under two minutes into the half when a testy Powling box kick from attacking ruck ball fell straight into the hands of Mike Groter.
The nippy wing broke through three lines of flat-footed Tenby defence, before possession was moved out through several pairs of hands to skipper John who touched down under the posts.
Liptrot added the extras, but Powling was unlucky not to reduce the arrears seconds later when he was off target with a penalty.
The try seemed to spark Cardigan into action, with a break from deep by Mathias almost netting Williams a try on the overlap, but referee Mr. Neil Howells called him back for a forward pass.
Such moments of incisiveness were rare in the second half, however, with much of play being a hard fought slog for possession between the packs.
Tenby lost the services of Duffy and veteran prop Simon Griffiths going into the final quarter, with Peter James and Gareth Noot coming on as replacements.
To their credit, what possession they secured, Tenby looked to move wide at every opportunity, but all too often the ball carrier was isolated or they were let down by handling errors.
One such error almost netted Cardigan another try when Powling fumbled in midfield and John intercepted.
The Cardigan skipper linked with Williams, but the youngster did not have the pace to skirt the tremendous covering defence of James and Dean Bowen.
The visitors were awarded a penalty at the breakdown, but Liptrot sliced the kick wide.
After looking under pressure in the first half, the Cardigan eight began to exert more control in the scrums, taking two set-pieces against the head, but they, too, were thwarted by handling errors and hard-hitting defending, which centred most of the play between both 22s for much of the final third.
A successful penalty from Liptrot looked to have sewn things up seven minutes from time, but the United refused to throw in the towel, driving into the visitors' 22.
The Cardi defence rallied to hold a concerted Tenby effort off a tap penalty to gain the put-in at a scrum.
Lloyd relieved the pressure with a kick to touch, but Tenby were not to be denied.
New Zealander Matt Cammack took the ball cleanly at the lineout, setting up a drive which saw the United roll possession on a full 20 yards for hard-working back row man Justin Richards to claim a touchdown.
It turned out to be the last hurrah for the Seasiders, however, Powling's failed conversion being the last kick of the game.
Tomorrow (Saturday), Tenby welcome Cwmavon to Heywood Lane.
Tenby United fielded: Dean Bowen, Nicky Rees, Paul Luger, Gavin Brace, Gareth Jones, Neil Powling, Neil Duffy, Lee Tucker, Richard Rossiter, Simon Griffiths, Matt Cammack, Paul Broaders, Justin Richards, Nicky Allen (capt.), Chris Brace.