Play had entered the first of three added-on minutes when James Mace applied the finishing touch to Jack Edwards’ header to settle a cagey affair.
And the jubilant scenes as the players ran over to celebrate with the home fans were born almost as much out of surprise as jubilation, with visiting keeper Glyn Garner having been called on to make just one routine save in the preceding 90 minutes.
Opposite number Tony Breeden was similarly underworked, with a point-blank stop from Kyle McLaggon in the fourth minute his only serious cause for concern.
It followed a spirited opening from the visitors, with a Brakes side robbed of the services of Darren Pond due to a groin strain and with Joe Magunda restored to midfield, struggling to clear their lines.
However, once they weathered the storm, it was the home side who were looking the more dangerous, even if that did not extend to testing Garner.
Three times Leamington cleared the Harbury Lane End stand with shots as they struggled to turn possession into meaningful opportunities.
When a swift counter involving Ryan Rowe and Thompson-Brown teed up a shooting chance for Ben George, he was too precise in his deliberation and his strike was blocked long before it got the chance to fly off into the night sky.
Merthyr were similarly wayward with their shooting, with McLaggon lifting the ball over the bar after Keyon Reffell had cut the ball back from the byeline and Reffell putting a first-time shot well wide a minute before the break.
Breeden had to be alert to block a fierce Ian Traylor drive at his near post in the 56th minute while Garner was drawn into a routine save from Ahmed Obeng after the winger had produced a superb first touch to bring down Thompson-Brown’s raking crossfield ball.
Courtney Baker-Richardson was brought on in place of George on the hour-mark and shortly after his introduction, Obeng teed up Ryan Rowe who was denied by a brave block from a defender on the edge of the box.
Town’s main threat was now coming from a series of corners, with captain Ashley Evans getting his connection all wrong in front of goal after ghosting in unmarked on one of his side’s set plays.
Going into the closing stages Holleran threw on Lee Moore and Rob Elvins, while Gavin Williams also exhausted his options from the bench as both sides looked to break the deadlock.
A bustling run from the imposing Ryan Prosser set up a chance for Reffell but Mace was across well to block him, while at the other end a superb last-ditch intervention from Adam Davies deflected Baker-Richardson’s goalbound shot into the side netting.
Elvins headed the resulting corner well wide, while Richard Taundry was unable to make the most of a promising free-kick 25 yards out despite his initial shot bouncing back off the wall to him.
The tempo of the game increased in the final five minutes as space opened up and a quick break from Leamington saw Obeng cynically chopped down in full flight by Davies to earn the defender a yellow card.
After treatment for Obeng, Thompson-Brown stepped up to whip in the free-kick and Edwards outjumped Garner to divert the ball goalwards.
It was not a clean connection but the ball still appeared to be heading over the line, with Mace applying an extra flourish to eliminate any element of doubt.
A bout of shoelace tieing ensured there was very little additional time to be played, with referee Darren Wilding’s final whistle heralding a fourth successive maximum for Brakes.
The action did not stop there, with a bout of pushing and shoving in the far corner between Baker-Richardson and Ashley Evans seeing a flurry of players from both sides keen to get involved.
However, temperatures quickly cooled to leave Leamington as the only side in the division with the warm glow that comes from a 100 per cent start to campaign.
Brakes: Tony Breeden, Richard Taundry, Connor Gudger, Joe Magunda (Rob Elvins, 83), Jamie Hood, James Mace, Ben George (Courtney Baker-Richardson, 60), Jack Edwards, Ryan Rowe (Lee Moore, 75), Rob Thompson-Brown, Ahmed Obeng. Subs not used: Tom James, Ryan Quinn.
Merthyr Town: Glyn Garner, Adam Davies, Scott Barrow, Gethin Jones, Curtis McDonald (Corey Jenkins, 53), Rhys Baggridge, Jon Brown, Ashley Evans, Kyle McLaggon (Elliot Scotcher, 78), Keyon Reffell, Ian Traylor (Ryan Prosser, 63). Subs not used: Jarrad Wright, Jaye Bowen.