Aarnio was a volunteer during the Finnish Civil War with White Guards at the Savonia Front. Later he participated to the Estonian War of Independence, the Latvian War of Independence and the Aunus expedition in East Karelia. Aarnio graduated from the Finnish military service in 1920 as a reserve officer. He graduated from the Kadettikoulu in 1926 and the Sotakorkeakoulu in 1933. Aarnio was promoted as captain in 1929 and served in the Foreign relations department of the General Staff.
During the Winter War, Aarnio was promoted to the rank of Major and he commanded the 4th Jaeger battalion (JP 4) in north of Lake Ladoga. His battalion became known for battles against encircled Red Army troops in the battle of Lemetti. During these motti battles, Aarnio used his eight best battalions, including his own battalion. The tactics of Aarnio was to destroy mottis at night time and in close range which proved successful. Later he received the nickname "Motti-Matti".
During the Continuation War, Aarnio was the Commander of the 9th Jaeger Regiment. His regiment was successful during the Reconquest of Ladoga Karelia. Aarnio was then appointed the Commander of the 56th Jaeger Regiment in December 1941. Later in the Lapland War, he was the Commander of the 2nd Jaeger Battalion.
- JR 56 ja JR 60 KILTA ry: MATTI ARMAS AARNIO – "Motti-Matti"
- While in Caracas, Aarnio assisted Finnish ex-patriot and Mannerheim Cross winner Lauri Törni as Törni was illegally immigrating to the United States in 1950. See: Cleverley, J. Michael (2008). Born a Soldier, The Times and Life of Larry Thorne. Booksurge. pp. 184–86. ISBN 978-1439214374. OCLC 58340971. and Gill III, Henry A. (1998). "8: Adrift". Soldier Under Three Flags. Ventura, CA: Pathfinder Publishing. p. 96. ISBN 0934793654. OCLC 38468782.