The experienced former Tusker defender James Situma grabbed the opener for the 'Slum Boys', Charles Odete scored an own goal before Harun Junior completed the route to help the team bounce back from last weekend's 2-1 defeat against Kakamega Homeboyz.
The Francis Kimanzi led charges had gone for three games without a win. They needed to make a statement with three points that took their season tally to 34 from 19 matches.
Posta Rangers tried a late comeback, but the John Kamau coached side could only pull a consolation through Calvin Odongo's second-half strike.
Mathare United XI: 1.David Okello, James Situma, Martin Ongori, Harun Junior, Andrew Juma, Roy Okal, Cliff Nyakeya, Chrispin Oduor, John Mwangi, Kevin Kimani and Clifford Alwanga.
Subs: Job Ochieng’, Arnold Onyango, James Kinyanjui, Alphonce Ndonye, Tyson Otieno, Ronald Reagan and David Owino.