Debutant John Mwangi scored Mathare United’s lone goal with a minute left to the break after delivering home a fine close range header after Tusker failed to clear the ball inside the box.
The win enabled the ‘Slum Boys’ to take their point tally to 23, three above second-placed Gor Mahia, who have three games in hand.
Mathare could have doubled their lead just a minute later but Tusker custodian, David Okello saved the moment to deny Edward Seda.
Seda missed the opportunity to extend the visitors lead when he opted to pass the ball to Cliff Nyakeya instead, of taking a direct shot with Okello way off his line but the goalkeeper gathered the curled pass.
Francis Omondi picked a yellow card in the 27th minute following a dangerous challenge on Boniface Muchiri, just seven minutes after he forced Eric Ambunya to pick Tusker’s first card in the 20th minute.
The win was a first for Clifford Alwanga, who was facing his former side for the first time after he was deemed surplus to requirements last December.
Tusker XI: David Okello, Kasadha Lawrence, Eric Ambunya, Lloyd Wahome, Shivachi Collins, Hashim Ssempla, Apollo Otieno, Timothy Otieno, Boniface Muchiri, Robert Achema and Edwin Lavatsa.
Subs: Duncan Ochieng, Eugene Asike, Nzuki Peter, Khamati Mike, Marlon Tangauzi and Asembeeka Amos.
Mathare United XI: Robert Mboya (GK), David Owino, Samuel Olwande, George Owino (C), Johnstone Omurwa, Francis Omondi, Edward Seda, Chrispin Oduor, Cliff Nyakeya, John Mwangi and Clifford Alwanga.
Reserves: Job Ochieng’ (GK), Gilbert Osonga, Roy Okal, Ahmed Ahmad, John Mwangi, Ronald Reagan and Klinsman Omulanga.