Karl was born on January 24, 1803, in Vienna[1] as the son of landowner Count Joseph Johann Anton von Thun und Hohenstein (1767–1810) and Countess Eleonore von Thun und Hohenstein, née Fritsch (1775–1834), and younger brother of politician Joseph Matthias von Thun und Hohenstein (1794 –1863).

[2] In 1840 he became a major and in February 1848 was made colonel[1] and commander of the 3rd Infantry Regiment.

[2][3][1] In the Second Italian War of Independence he commanded the 15th Army Corps, which was newly formed to defend the coast, but was not deployed.

[2][1] In the Austro-Prussian War his corps was part of the army of Feldzeugmeister Ludwig Benedek, serving in Bohemia and being involved in heavy fighting in the Swiepwald.

[4][1] He had been married to Johanna née Freiin von Koller (1809–1891) since March 7, 1833.