The 66th Military Intelligence Group was activated at Camp Rucker, Alabama on 1 July 1944 as the 66th Counter Intelligence Corps Detachment. Attached to the 66th Infantry Division, the 66th CIC Detachment deployed to France on 27 December 1944. The Detachment's primary mission was to screen French refugees fleeing from besieged German strongholds at St. Nazaire and Lorient, France. After a short post war service in Germany, the unit returned to the United States and was inactivated at Camp Kilmer, New Jersey on 12 November 1945.
Four years later, the 66th CIC Detachment was reactivated and assigned to the United States Army Europe in Stuttgart, Germany and in December 1952, redesignated as the 66th Counter Intelligence Corps Group. Its primary mission was to conduct counter intelligence operations in the southern German states of Baden-Wuerttemberg, Bavaria, Rhineland Pfalz and the Saarland.
On 1 April 1962, the unit reorganized with newly activated security, collection and linguist companies to support the Seventh US Army. As a result of further consolidation of Army intelligence assets in Germany during September 1968, the 66th MI Group moved from Stuttgart to Munich and assumed the additional Counter Intelligence mission in Northern Germany, as well as intelligence support to the United States Army Europe.
In February 1977, the 66th MI Group was assigned to the US Army Intelligence and Security Command and on 16 October 1986, was redesignated as the 66th Military Intelligence Brigade. After the collapse of the Warsaw Pact and the Soviet Union, the Brigade once again restructured and relocated to a new headquarters in Augsburg, Germany with subordinated detachments operating throughout Europe. In Summer 1995, a drawdown coupled with treaty restrictions resulted in the inactivation of the 66th MI Brigade. It was replaced by the 66th MI Group and eventually relocated to Darmstadt, Germany due to realignment and base closures.
In 2002, the 105th MI Battalion (Provisional) was added to the Group organization and the 533rd MI Battalion was deactivated. The 2nd MI Battalion was reactivated 17 October 2002 in Darmstadt, Germany. The 109th MI Battalion (Provisional) at Menwith Hill Station, England was assigned in 2002 and was re-designated the 307th MI Battalion in 2008. On 15 September 2005, the 323rd MI Battalion (US Army Reserve) at Ft. Meade, Maryland was aligned with the Group. In late 2006, the 1st MI Battalion (Aerial Exploitation) was assigned to INSCOM and then permanently attached to the 66th MI Group. With the deactivation of the 205th MI Brigade, the 302nd MI Battalion was assigned to the Group in January 2008. In July 2008, the 66th MI Group was re-designated as the 66th MI Brigade and in September of that year, the Brigade moved from Darmstadt to Wiesbaden Army Airfield, Germany.
2009 saw further transformation as the 307th MI Battalion was re-designated the 709th MI Battalion, 302nd MI Battalion was inactivated, and the 24th MI Battalion was activated. Soldiers of the 66th MI Brigade have made long lasting contributions during World War II, the Cold War, Operation Desert Storm, the Balkans, and Operations Iraqi Freedom and Enduring Freedom.