Chris Ochieng scored the first goal of the encounter in the 15th minute and the Sugar Millers were unable to find an equalizer. The goal marks Ochieng's first goal of the season and ensured the Slum Boys picked up maximum points against Chemelil Sugar.
The win also marked Mathare United's first victory at Chemelil Sports Complex in nine years. They now have 54 points and will face the league champions Gor Mahia on May 29 in the last match of the season.
Meanwhile, Sony Sugar and Nzoia United drew 0-0 at Awendo Green Stadium.
Sony Sugar captain Enock Agwanda failed to make a successful conversion from the spot when the hosts won a penalty in the 10th minute. A Benson Mwangala fingertip save is what kept Nzoia Sugar in the game, as Sony dominated from the first minute.
The visitors almost got the first goal in the 17th minute as Austin Ochieng, a former Sony Sugar man, quickly broke into the area but Samuel Njau was alert to deal with any danger presented by Ochieng.
The visitors were reduced to 10 men on the pitch in the 45th minute when Kevin Juma was given a straight red card after an awful challenge on his opponent.