After having been educated at the Theresian Military Academy, Giesl von Gieslingen was posted to a regiment of dragoons in 1875. Between 1877-1887 Giesl von Gieslingen worked for the Austrian military intelligence, the Evidenzbureau. In 1887 he became orderly officer to Crown Prince Rudolf. In the Mayerling incident, he was one of the few to see the dead bodies of the Archduke and his mistress Mary Vetsera. In 1891 he became adjudant to Emperor Franz Joseph I of Austria.