They found themselves trailing Maheno by four points with time up on the clock in Oamaru on Saturday.
However, Old Boys held the ball for an additional five minutes as they chipped away at the green machine’s defence.
Carry after carry got them closer before Tini Feke crossed to seal the 44-41 win.
Earlier, Maheno struck first through Sam McCarthy.
Feke then scored the first of his four tries to close the gap, before Ben McCarthy nailed a penalty to give Maheno the lead.
Inoke Fisilau ran away with an untouched 50m intercept try and Mason James added the tough conversion to give Old Boys their first lead of the game.
Feke then ran in his second try, before Maheno tied things up heading into halftime with a beautiful try of their own.
After securing a turnover, they spun the ball wide on the counter attack.
McCarthy ran a nice line and received an inside pass to put him through a gap.
He evaded another two defenders before firing a lovely inside pass to Alex Wilson who was in support and scored under the sticks.
The second half was an even contest as both sides traded tries.
A lineout drive put Sailosi Vea over for Old Boys before Maheno replacement March Balchin scored.
Feke’s third try was then countered with a penalty try for Maheno.
They were hot on attack close to the try line when Old Boys committed a professional foul, giving the men in green a free seven-pointer.
With less than 10 minutes to play, Old Boys’ lineout again proved a catalyst for a try as Sione Kavatoe crashed over off a driving maul.
Maheno then fought their way back to score through an excellent effort from Jack Newlands.
That put them in front and set up the grandstand finish.
It is the second week in a row Maheno has lost at the very end but they continue to show plenty of improvement from last year.
The win could prove vital for Old Boys who have been up and down in 2026 so far.
Competition front-runners Valley dispatched Athletic Marist 47-7 to stay undefeated.
They ran in seven tries to one in a dominant display.
Excelsior also got an important win over Kurow at home.
The defending champs downed the Red Devils 57-21.
