Joseph Johnston was born on 28 February 1745 in Dublin, Ireland. He first appears on Virginia's Colonial Military Soldiers list as being stationed at Fort Fincastle in 1774 during Lord Dunmore's War.

Records indicate that Joseph enlisted on 10 April 1775 and served in Captain Joseph Spencer's Company. Often the record refers to his later service as a Sergeant in the 7th Virginia Regiment, but this unit stood up later in the war. 

His initial service would have been as a Soldier in the Culpeper District minute battalion of 1775. He may have been one of the 150 men under Spencer's command who participated in the Battle of Great Bridge on 9 December 1775.

The 7th Virginia regiment was taken into Continental Service on 29 February 1776. The regiment closed with the main Army under Washington by January 1777. Assigned to Brigadier General Woodford's Brigade, Stephen's Division, Sullivan's Wing, the Regiment suffered heavy casualties fighting against British light infantry and German Jaegers at the Battle of Brandywine on 11 September 1777.

The 7th Virginia was next in action on 4 October 1777 at the Battle of Germantown. Now under the Wing command of Nathanael Greene. Greene's column on Limekiln Road to the northeast and had a long march to the battle area. Greene's vanguard engaged the British pickets at Luken's Mill, driving them back after a savage skirmish. The fog clung to the fields. This was compounded by palls of smoke from the cannon and musket fire, throwing Greene's column into disarray and confusion. Brigadier General Adam Stephen's brigade, including the 7th Virginia, veered off-course and began following Meetinghouse Road, instead of rendezvousing at Market Square with the rest of Greene's troops. The wayward brigade collided with Wayne's brigade and mistook them for redcoats. The two American brigades opened fire on each other in the fog, causing both to flee the area in disorder.

Joseph would march into the Army's winter encampment at Valley Forge on 19 December 1777, one of 427 assigned men, but it is unknown if he was counted among the 46 who were fit for duty. He was discharged on 13 February 1778.

The Patriot died on 15 August 1825 in Friendsville, Blount County, Tennessee.