With the score still goalless with 10 minutes to go, it looked as though points were going to be shared but Tarxien's unrelenting pressure finally produced the deserved dividends in the 79th minute and piled more misery on Clove Mizzi's side whose limp display saw them crashing to their fourth consecutive defeat.
Substitute Karl Pulo with virtually his first touch of the ball prodded in his side's winner with composed finishing. The assist came from Ricardo Costa, who, for the umpteenth time this season, showed that there is more to his game than scoring goals. Upping the tempo in the second half, Ricardo jinked his way past three defenders before crossing towards the far past where Pulo flashed in to power the ball home.
Inside the first two minutes, Kris Calleja's double block denied Mayasi and de Castro after the Rainbows had worked a slick passing move across their opponents' penalty area.
Mqabba had the ball in the net after 28 minutes but Bueno's header was ruled out for offside.
Once again, Calleja came to Mqabba's rescue, saving from Mayasi with his legs and the effect of this miss appeared to puncture Tarxien's self-belief.
Having received a third warning, Mqabba took control of the game when Thiago Mazzitelli got in front of Mamo to steer the ball into the side-netting with the sole of his boots just after the half hour mark.
After 41 minutes, David Azzopardi saw his menacing shot ably punched into a corner by Tarxien goalkeeper Nicky Vella.
Tarxien's confidence had clearly been boosted by their first-half performance and they started the second period much more positively.
Tanti volley narrowly in the 47th minute then had an even better chance to score four minutes later when Bueno's brilliance zipped open the defence as he deposited the ball towards the unmarked Mayasi. When the Dutch striker found himself face to face with Calleja, skewed his shot wide.
On the hour mark, Mqabba's tight defence was unlocked by a neat three-man move, de Casto pinged the ball in the space into which Mayasi intelligently run, before delivering an inviting ball into the box but Bueno shot straight at Calleja when well placed.
Three minutes later, Costa broke free on the right-hand flank, worked his way to the Mqabba box and crossed for de Castro whose firm volley crashed against the bar.
Patrick Curmi moved to strengthen his attack introducing Adrian Carabott and Karl Pulo for Mayasi and Tanti respectively.
In the 73rd minute, Timotic superbly cleared off the line to deny de Castro from free-kick.
The Rainbows finally grabbed the lead in the 79th minute when Pulo burst into the area from the left wing to meet Costa's delivery and fired a precise shot that beat Kris Calleja to his right and went in despite Bonnici's desperate attempt at clearance.