Upon announcement that Oklahoma and Indian Territories were to combine into one state, Johnston was elected in 1906 to represent Noble and the surrounding counties at the Oklahoma Constitutional Convention. During the convention, Johnston would be elected to serve in the body's number-two office as the President Pro Tempore of the Convention. Johnston met future governors Charles N. Haskell, William H. Murray and Robert L. Williams. These men would work together to write one of the most progressive Constitutions of any U.S. state, as well as the longest governing document in the world at the time.