He was the youngest son of Ferdinand Bonaventura I von Harrach (1637 - 1706), an important Austrian diplomat during the reign of Emperor Leopold I and Imperial Ambassador in Spain.
His elder brothers were Franz Anton, who became Bishop of Vienna and Archbishop of Salzburg, and Aloys Thomas, Viceroy of Naples.
During the retreat, he held the advancing French army with his Herberstein regiment and six grenadier companies at the bridge over the Chiese River, until his own defeated troops could be regrouped and brought back into action.
After winning the Battle of Turin (7 September 1706), Prince Eugene sent him to Vienna with the news of his victory, because of his services during the campaign.
in 1739 Emperor Charles VI appointed him President of the Imperial War Council, an office he held during 23 years, until 1762.