Regiment and assumed command of it in 1803.  As a brevet Lieutenant Colonel commanding the 20th, Ross saw significant action in Spain, Egypt, Italy, and the Netherlands.  He was wounded 3 times, two of which were severe.  For his conspicuous gallantry, leadership, and heroism, he was awarded three Gold Medals, the Peninsula Gold Medal, a Sword of Honor, and he received the thanks of Parliament.
Ross was a seasoned veteran of the Duke of Wellington's campaigns and a strict disciplinarian who drilled his men relentlessly. Nevertheless, he was very popular with his men.  He was always ready to share in his soldiers' hardships and fight alongside them in the thick of battle - as attested to by his three wounds.  In 1812, Ross was promoted to major general.
When the Napoleonic War ended, Wellington sent Ross to North America to unite with Royal Navy units under the command of Rear Adm. George Cockburn and Vice Adm. Sir Alexander Cochrane. He commanded a 4,500-man army.  The mission of their combined forces was to divert the attention of American forces from other theaters of the war by raiding the coast of North America.
The military action Ross is most widely know for is the capture and burning of Washington, D.C. on 24 August 24.   This followed the Battle of Bladensburg, where American militia forces broke and ran before the attacking British Infantry, remembered in history with the title of the "Bladensburg Races".  The only real defense offered by American forces were 500 flotillamen and 120 Marines, manning 3 heavy Navy cannons and 3 lighter field artillery pieces, commanded by Commodore Joshua Barney.  Ross, again leading from the front, had his horse shot out from under him while directing his regiments in the eventual capture of the cannons and a badly wounded Barney.  Later the same day, Ross had a second horse shot from under him when he entered the town of Washington.  On 26 August 1814, Ross and his men embarked their ships and sailed North in the Chesapeake for Baltimore.
On a moonless night from 3:00 A.M. to 7 :00 A.M. on September 12, Ross and his men began landing on the North Point Beachhead.  Still in high spirits and morale from their capture of Washington less than 3 weeks earlier, they formed into march columns for the assault on Baltimore.  The column stretched from the beachhead to Todd's Inheritance.  Patrols were dispatched throughout the lower North Point Peninsula to reconnoiter for the defending Americans. 
It was a hot morning and the British soldiers were severely fatigued by the march. Ross halted his men beyond the Gorsuch Farm. British flank security elements captured and brought in 3 American soldiers.  When questioned by Ross about the American defenses, the 3 prisoners told him Baltimore was defended by 20,000 soldiers and 200 cannons.  He reportedly laughed and said, "But they are mainly militia, I presume".  Having almost scattered what he believed to be similar militia at Bladensburg, he said he'd take Baltimore, "if it rains militia!".  Afterward, Mr. Gorsuch asked sarcastically if they would be returning for supper.  Reportedly, Ross replied, "I'll sup tonight in Baltimore - or Hell".  What the captured cavalrymen cleverly failed to mention was that more Americans waited near by.
The American field commander, Brig. Gen. John Stricker r and his brigade of just over 3,100 men had been ordered down the North Point Peninsula by Maj. Gen. Samuel Smith, the commander in chief during the Battle of Baltimore.  Smith was convinced that the British would attack Baltimore via the North Point Peninsula and had prepared his defense accordingly.  When he learned conclusively that the British fleet had assembled in the Patapsco River at the mouth of Old Roads Bay, Stricker's brigade was ordered to meet them and fight a delaying action.
Several American officers volunteered to move forward and dislodge the British from the Gorsuch Farm.  Maj. Richard Heath, with 2 companies of the 5th Maryland Regiment, and Capt. Edward Aisquith's rifle company, with one cannon, about 230 men in all, moved forward.
Soon after midday, as Ross and his staff were riding through an area locally know as Godly Wood, the British 4th Light Infantry Regiment made contact with the men commanded by Heath.  Riding to the sound of musketry to be at the front with his men, Ross had mingled with the skirmishers and was shot through the right arm into the chest.  He fell into the arms of his aide-de-camp, Capt. Duncan McDougall, and was moved to the rear and laid beneath a tree on Robert Gorsuch's farm.  He lived long enough to name his wife and commend the protection of his family to his country.  Two Americans, Dan Wells and Henry McComas, are credited as the sniper team that killed Ross, although both were killed in the engagement. Ross died within minutes.
Credit for the killing of Ross is given to Privates Daniel Wells, 19, and Henry G. McComas, 18.  Both were members of Aisquith's Rifle Company and died at the same time that Ross was shot.
Ross' body was taken to Vice Adm. Sir Alexander Cochrane's flagship, H.M.S. Tonnant, and stored in a barrel of 129 gallons of Jamaican rum to preserve it for the return to his home in Ireland.  However, due to impending operations at New Orleans, his body was transferred to the H.M.S. Royal Oak and he was buried on September 29, 1814, with full military honors at St. Paul's Church in Halifax, Nova Scotia. 
It is thought that preparations for the Battle of New Orleans prevented his body from being shipped back to Britain.