Ehrnrooth entered cadet school in 1922 and served in the Uusimaa Dragoon Regiment (Uudenmaan Rakuunarykmentti).
During the Winter War he served on the staffs of the 7th Division and the Cavalry Brigade. From the beginning of the Continuation War he served as the chief of staff of the 2nd Division until he was seriously wounded. After he recovered he was appointed to lead the 7th Infantry regiment (JR 7) of the 2nd Division. During the battles on the Karelian Isthmus he was awarded the Mannerheim cross.
After the wars he led an active military career until he retired in 1965.
Adolf Ehrnrooth is the face and voice most associated with rehabilitation of the fighters who secured Finland its independence. The long era during which it simply was not progressive to value the military ended in the early 1990s, at which time his charismatic persona was at its height.
He became a figurehead for the whole veteran community—straight-talking and unapologetic. When he found out that many nationalist youths cited him as an idol, he blasted them squarely by saying that he might have in his own youth held nationalistic views, but that Finland's path was now in the European Union, a view the nationalists did not hold.
Pundits have found his idiosyncratically-emphasized speech patterns and overheavily guttural snarls a rich ground to harvest. For them he is one of the indispensable voices.
In his last statement he gave support to ProKarelia (Finnish NGO) and its plan for the Return of Ceded Territories. He said that he defended the borders of Finland declared in the Treaty of Tartu and considered them the only proper borders of Finland and that it was great injustice that the Soviet Union had taken these territories.
In 2004, General Ehrnrooth was voted as the 4th greatest Finn of all time by the Finnish public during the Suuret suomalaiset (Great Finns) competition.