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.
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.
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.