At the outbreak of the French Revolution, Tharreau enthusiastically adopted the revolutionary cause. In 1792, he joined the Maine-et-Loire volunteers and later that year he was an adjutant major of the 2nd battalion of volunteers of Mayence. In 1794, at the height of the French terror, he was a General of Brigade in the Army of the Ardennes. In 1799, Jean Victor Tharreau was a brigade commander in the French Army of the Danube, and fought at the battles of Ostrach and First Stockach. On 20 April, he was promoted to General of Division. André Masséna placed him command of the forward line, for the defense of the Swiss city of Zurich, in 1799; the insubordination of Jean-de-Dieu Soult at the Battle of Winterthur in May 1799 contributed to Tharreau's inability to hold the forward line, forcing a French retreat on Zurich and the subsequent defeat of the French at the First Battle of Zurich on 4–7 June.