Born in Vienna into the noble family "Festetics de Tolna", Tassilo Festetics was the grandson of the agricultural pioneer György Festetics. In 1827 he joined the Chevauxlegers Regiment No. 2 as a second lieutenant. He served in the army until he left active servive on 15 February 1846. Recalled he took part in the suppression of the Kraków uprising and in the campaign in Italy in 1849. On 31 July 1849 he returned to active service as a lieutenant colonel and from September 1849 to July 1857 he commanded Hussar Regiment No. 7.