In 1968, Sieben joined the
United States Army in the Military Intelligence Branch. He completed basic and advanced training at
Fort Dix, New Jersey, and
Fort Devens, Massachusetts. From 1969 to 1975, Sieben was stationed at
Fort Snelling, Minnesota. During this period, Sieben worked as a
Morse code intercept operator and platoon leader in the
Army Security Agency (ASA), the Army's signals intelligence branch. He then received a direct commission into the Minnesota
Air National Guard in April 1975. He served in the
Judge Advocate General's Corps for the 133rd Airlift Wing, 148th Fighter Wing, and Headquarters Minnesota Air National Guard. In 1990, he was appointed as the state judge advocate for the Minnesota National Guard. In 1997, he was promoted to Assistant Adjutant General in the Minnesota Air National Guard based in St. Paul. In this position, Sieben was responsible for liaison with senior leadership at higher headquarters, coordination with state and federal agencies, direction of long range planning, coordination of real estate matters, and implementation of plans and programs. In October 1998, Sieben was promoted to brigadier general at National Guard state headquarters in St. Paul. Major General and Adjutant General Eugene Andreotti, and Sieben's uncle and former Adjutant General James Sieben, pinned the stars on Sieben's shoulders at the ceremony. In August 2003,
Governor Tim Pawlenty appointed General Sieben, then 59, as the acting
Adjutant General of Minnesota, the state's top-ranking officer. During Sieben's tenure, he was responsible for the 12,500 men and women of the Minnesota National Guard. While Sieben was acting Adjutant, about one-fourth of the state's total force was activated, or soon to be activated, including service in places to include Iraq, Bosnia, and Kosovo. After retiring from the military, Sieben was invested as Civilian Aide to the Secretary of the Army for Minnesota in October 2006 at
the Pentagon by
Secretary of the Army Francis Harvey. Sieben's awards and decorations include a Meritorious Service Medal, Air Force Commendation Medal, Air Force Outstanding Unit Award, Air Force Organizational Excellence Award, and Small Arms Expert Marksmanship Ribbon. Sieben also received the Minnesota Superior Volunteer Service Award. ==Legal career and other work==