[1] A lawyer by training, Johnston became district attorney of Westmoreland County at the age of 21 in 1829.

Johnston fought the federal Fugitive Slave Act and its enforcement in Pennsylvania.

In 1856 he was nominated by the northern, anti-slavery faction of the American Party for the office of Vice President, but was later induced to withdraw in favor of William L. Dayton, the Republican nominee.

[3] In 1864, in the midst of the American Civil War, Johnston refused to support the renomination of incumbent president Abraham Lincoln by the Republican Party, instead backing the splinter Radical Democracy Party and their campaign in favor of John C. Frémont, the Republican Party's presidential nominee in 1856 who was now campaigning on a platform calling for a more radical reconstruction than Lincoln endorsed.

His father built and operated the Kingston House in Unity Township, Pennsylvania.