First half goals from James Situma, Junior Harun and an own goal from Posta Rangers captain Charles Odette helped Mathare United register a 3-1 win in a SportPesa Premier League match at the Kenyatta Stadium, Machakos on Thursday.
It was the Slum Boys' first win of the second leg campaign.
Mathare went ahead in the 15th minute when Odette - who had the armband in the absence of Jockins Atudo- put the ball into his own net in an attempt to clear a John Mwangi cross from the left flank.
Veteran Situma then made it 2-0 in the 29th minute by nodding home a Kevin Kimani free kick.
Six minutes later, Harun Junior, who was handed a rare start by coach Francis Kimanzi, fired home the third goal after pouncing on a loose ball inside the penalty area.
John Kamau's Posta showed massive improvement after the break, but failed to make their chances count.
The Mail men finally found the consolation goal through Cavin Odongo, whose sumptuous volley, easily beat Mathare custodian David Okello on 73 minutes.
It was, however, a little bit too a late for Kamau's charges as the Slum Boys protected their advantage to leapfrog Bandari and move to third position on the log with 34 points.
Posta are third from bottom on 16 points after 19 rounds.