ODM Secretary General Edwin Sifuna said he will deliver to the people of Nairobi and do the work as expected of him.
He won the Nairobi Senator seat after garnering 716,876 votes.
UDA's Bishop Margaret Wanjiru came second after she got 554, 091 votes.
Speaking after receiving his certificate, Sifuna expressed gratitude to the people of Nairobi for voting him in.
"Thank you for honoring me with this particular position and I want to assure you that you know my capacity, you know my work, my ability, I will serve you diligently and will wake up to work for the people of Nairobi," he said.
He also thanked his team that was always helping during the campaigns and his family.
"I am proud to say that I have gone through this entire campaign without mentioning any of my opponents but I consider them worthy opponents, and they are welcome to work with us in order to deliver for the people of Nairobi," he said.
Nairobi County Returning Officer Albert Gogo Nguma issued Sifuna with a certificate amid jubilations at the Kasarani Sports.
Sifuna will now take over from Nairobi senator Johnson Sakaja who is running for the city's governor seat.
