Obiero, who is continuing to have good times since making his Kariobangi Sharks debut, made two quick saves in the 19th minute to deny Mathare s Lennox Ogutu and David Ochieng.
Kariobangi Sharks almost doubled the lead in the 25th minute when top scorer Erick Kapaito collected a poor clearance by Mathare s backline but his effort was stopped by the woodwork.
Lwasa, however, added the second goal for the visitors a minute past the half-hour mark with a low drive to punish a rather disjointed Slum Boys.
In the 52nd minute, Ochieng s free-kick flew just above the crossbar as Mathare United made their first attack of the second half.