Damjanich entered the army as an officer in the 61st regiment, and on the outbreak of the Hungarian Revolution of 1848 was promoted to be a major in the third Honvéd battalion at Szeged. Although an Orthodox Serb, he was from the beginning a devoted adherent of the Hungarian liberals.
His ability and valour at the battles of Alisbrunn (Serbian: Alibunar, Hungarian: Alibunár) and Lagerdorf/Temesőr in 1848 led to his promotion to colonel. In early 1849, he was appointed commander of the 3rd Army Corps in the middle Tisza, and quickly gained the reputation of being the bravest man in the Hungarian army. In March 1849 he annihilated an Austrian brigade at Szolnok, which was perhaps his greatest exploit.
The engagements of Hort and Hatvan, along with the bloody Isaszeg turned Damjanich into a national hero. At the ensuing review at Gödöllő, Lajos Kossuth expressed the sentiments of the whole nation when he doffed his hat as Damjanich's battalions passed by. Damjanich uncompromisingly supported the views of Kossuth, and was appointed commander of one of the three divisions which, under Görgey, liberated Vác in April 1849. His fame reached its height when, on April 19, he won the Battle of Nagysalló, which led to the relief of the fortress of Komárom.