Mbarire, who is also Embu governor, said the defection meant an end to the infighting that has rocked the county for the past year.
Meru Governor Kawira Mwangaza has aligned herself with President William Ruto's ruling party, the United Democratic Alliance (UDA).
Governor Mwangaza was received by UDA Secretary General Cleophas Malala and party chairperson Cecily Mbarire at the party's headquarters in Nairobi on Wednesday.
