His political image was further bolstered by his trouncing of such political heavyweights as Dick Wathika and Reuben Ndolo to clinch the seat. The September 20, 2010 by-election was occasioned by a successful petition by Reuben Ndolo.
After the inauguration of the new constitution of Kenya in 2010 and the creation of the county governments, Sonko announced his intention to run for Senator in Nairobi County. He became the First Senator of Nairobi after it was announced that, with 808,705 votes, he beat his closest competitor, Margaret Wanjiru of the Orange Democratic Movement, who had garnered 525,822 votes, in the Nairobi senatorial election of 2013. Unlike his time in parliament, where he gained notoriety for flaunting House Rules, including incidences of inappropriate dressing.
During his time in the senate, he formed the Sonko Rescue Team, an officially registered Non-Governmental Organization that provided government-like services to residents of Nairobi slums. The then governor of Nairobi County, Dr. Evans Kidero opposed the activities of the group, citing conflict of interests with the county government’s operations. The Senator acquiesced and donated the vehicles to the county government after several weeks of tense public exchanges.
As early as January 2016, Mike Sonko had announced his intentions to run for Governor of Nairobi County in the Kenyan general election, 2017. After a grueling primary campaign against Peter Kenneth, Sonko run for the Gubernatorial seat for Nairobi County with the Jubilee Party of Kenya and defeated the incumbent Evans Kidero to become the second Governor of Nairobi.