As a young man in the 1890s he became a supporter of the Finnish independence. His father changed the original, Swedish, family name Johansson to Airo (lit. "oar") alongside thousands of others in 1906.
During the Civil War in Finland (1918), Airo served in the artillery on the White side, taking part in battles near Viipuri. At the end of the war he was already a lieutenant. Afterwards he was trained as an officer in Lappeenranta artillery school and was sponsored to the French military academy, Ecole militaire in St. Cyr in 1920. 1921 he was accepted into Ecole superieure de guerre, the French officer training academy, from which he graduated as a captain in 1923, at the age of 27. Mannerheim invited him to join Finland's Defense Council as a secretary.
Airo rose swiftly in ranks, mainly because the newly independet Finland needed suitable officers for the fledgling army. He got, however, some problems with the fact that he was neither a Germany-trained Jaeger officer, nor one of the officers trained in the Tsar's army during the Russian rule. Still, by 1930 he had become a colonel.
In the beginning of the Winter War, Mannerheim appointed Airo as Quartermaster-General, and he was promoted to a major general, and two years later to lieutenant general. On November 18th, 1944, Marshal Mannerheim made him a Knight of the Mannerheim Cross, Cross of Liberty.
Airo was in the Mikkeli headquarters during the war and rarely went to field. He was responsible for operational planning and the presentation of operations, or, as he allegedly said "The Marshal leads the war, but I lead the battles". They had many differences in opinion but still managed to work well together.
After the end of the Continuation War, the now Communist-dominated Valpo (the Finnish State Police) arrested him for his alleged involvement in the so called Weapons Cache Case (hiding weapons in preparation for a guerilla war in case of Soviet takeover, which according to the Finnish Communists was a breach of the armistice). He was inprisoned from 1945 to 1948 without being sentenced, until president Juho Kusti Paasikivi released him. He said little about the affair afterwards and earned the moniker "the silent general". The President relieved him of his duties with a special permission to wear military uniform. He never reached the rank of full general.
In his later life Airo was a member of the parliament for theKansallinen Kokoomus party and a presidential elector. Unlike many of his contemporaries, he never wrote memoirs about his war experiences. In 1982 President Mauno Koivisto awarded Airo with the membership of the Finnish Order of White Rose. He died in his home farm 1985.
Airo was known for some recklessness and sarcastic humor. He also had taste for liquor, tobacco and ladies.