He edited the Houston Post from 1885 to 1919, and served a 29-day term in the United States Senate in January 1913 after the resignation of Joseph Weldon Bailey.

As a noted editorial writer, Johnston was frequently quoted by other newspapers across the United States.

[1] At the beginning of January 1913, Oscar Branch Colquitt, Governor of Texas, appointed Johnston to the United States Senate to fill the unexpired term of Joseph W. Bailey.

[2] Elected to replace Johnston was Morris Sheppard who would go on to introduce the Eighteenth Amendment to the Senate, implementing the nationwide prohibition of alcohol.

[3] Following his Senate term, Johnston returned to Houston and resumed his role as editor of the Post.

He resigned his seat in 1920, having been appointed by Governor William P. Hobby as Chairman of the State Prison Commission.

In a special election, Lynch Davidson, later a state Lieutenant Governor, won Johnston's former seat.