He enlisted in the Hellenic Army as a volunteer on 17 January 1904, and after studies at the NCO School, was commissioned as an Artillery Second Lieutenant on 7 July 1912 (O.S.).

[2] He participated in the Balkan Wars of 1912–13 as commander of a machine gun section, and was promoted to Lieutenant in 1913 and to Captain in 1915.

He served on the Macedonian front during World War I as a staff officer, being promoted to Major in 1918.

After his return to Greece, he served as Governor-General of Western Thrace in 1947–48, before being appointed as Chief of the Hellenic Army General Staff on 21 January 1949, a post he held until his retirement on 16 March 1951.

[2] Kosmas then entered politics, being repeatedly elected an MP and serving in the cabinet of Alexandros Papagos as Minister Governor-General of Northern Greece from 15 December 1954 to 24 May 1955, when he became the first Minister for Northern Greece, remaining in the post until 6 October 1955.