Upon his father's death in 1823, he traveled to Spain where he briefly fought in the liberal uprising against the absolutist rule of Ferdinand VII; before being taken prisoner by the French army. In 1825, he was among a group of volunteers recruited by Colonel Charles Fabvier in Marseille to fight in the Greek War of Independence. He arrived in Nauplion in 1825, initially serving in Colonel Auguste Regnaud de Saint-Jean d'Angély’s cavalry as a Trooper. Myhrberg took part in the fighting in Euboea in 1826, he then fought in an attempted relief of the Siege of the Acropolis where he was injured. In 1826, he was promoted to Lieutenant, later participating in the where he was wounded again. He was wounded in action for the third time in the Battle of Cape Colias, being promoted to Captain after its conclusion. In January 1827, Myhrberg became Fabvier's aide-de-camp subsequently serving in the same role for Major General Thomas Gordon and Colonel Carl Wilhelm von Heideck. Myhrberg took part in the Battle of Phaleron and the Chios expedition where he distinguished himself. In 1829, Myhrberg was appointed Commandant of the Palamidi fortress by Ioannis Kapodistrias. Apprehensive to the growing influence of the Russian Party, Myhrberg left Greece in 1831 in order to participate in the Polish November Uprising. Little is known about this period of his life, according to an anecdotal account by Anders Ramsay, Myhrberg fought at the Battle of Ostrołęka. He was captured by the Russians at some point during the war but managed to escape to neutral Prussia, later settling in France.