Kienmayer led the II Reserve Corps in the War of the Fifth Coalition in 1809. This 9,000-man formation comprised five battalions of grenadiers and 12 squadrons each of cuirassiers and heavy dragoons. His troops fought under Johann von Hiller's orders at the Battle of Landshut on 21 April. During the retreat, he also led his corps at the Battle of Ebersberg on 3 May.