On November 16, 1907, the United States Congress accepted the Oklahoma Constitution. On the same day, Charles N. Haskell was inaugurated as the state's first governor. Before the state constitution was approved, Johnston ran and was elected to the Oklahoma Senate to serve in the 1st Oklahoma Legislature. A popular figure, Johnston was selected to serve as the first President Pro Tempore of the Oklahoma Senate, the state senate's highest official behind the Lieutenant Governor of Oklahoma, who serves as President of the Oklahoma Senate.