Dimba won by 11,230 votes, beating his rival Frank Mazizi of DPP with a wide margin. He got 1,663 votes.
There were five contestants in total.
MEC Chairperson Justice Jane Ansah announced the results at a hotel in Blantyre, where she bemoaned low- turn at the bye election.
She said about half of the 32,495 registered voters turned up, representing 46.97 percent.
Ansah said MEC was satisfied with the vote, saying it was conducted in a "free environment without fear and intimidation."
The bye-election was held after one of the candidates during the tripartite elections of May 21, Agnes Penemulungu, died.
An attempt to hold the vote last October was aborted after violent clashes erupted between political parties.
Ansah said the vote was postponed to "save lives, property and give room for addressing challenges."