A letter doing rounds on social media indicates that Sonko has chosen Miguna Miguna as his Deputy.
The letter has blindsided Kenyans who have been doubting the veracity of the letter.
The governor of Nairobi has been in the process of looking for a deputy after the dramatic resignation of his deputy Polycarp Igathe.
The reactions of Kenyans to the development has been one of surprise and disbelief.
Here are the reactions:
''Somebody tell me this is fake news?' Jim Kariuki wrote.
''Sonko has dealt the mafia a blow to the nose. I can't believe this,'' Amos Amukune said.
''Wololololo! Surprises will never cease,'' Sarah Nanjala wrote.
''The plot has thickened. Popcorns please,'' Emmanuel Baraka said.
''It is true! Miguna Miguna will be the next Deputy Governor of Nairobi. Wueh!'' Amina Khatib wrote.
''This will rank as one of the most surprising political development in the history of the country,'' Salmon Ndirangu wrote.
''I don't know even what to say,'' Maggie Omollo said.
Here is the appointment letter: