The rebirth of IMU after World War II took some time. In December 1949, a Policy Committee made the decision that a Union Conference would be held in New York. This took place during August 27-29, 1950 and resulted in a draft of Statutes and By-Laws and an Enabling Resolution. In December 1950, the Statutes and By-Laws of the Union had received their final touch. The Enabling Resolution stated that the IMU would be established as soon as ten countries had joined. This happened on September 10, 1951. The IMU was again in official existence with the Danish Academy of Science as its first headquarters. The first ten members of the new IMU, in alphabetic order, were Austria, Denmark, France, Germany, Great Britain, Greece, Italy, Japan, the Netherlands, and Norway. Five more countries joined in 1951: Australia, Canada, Finland, Peru, and the United States. The first General Assembly took place in Rome in March 1952. The years 1950-1952 are milestones in the history of IMU. The Constitutive Convention in 1950 in New York created IMU de facto. By the Statutes adopted there, IMU came into being in 1951 de jure, and in 1952 the General Assembly inaugurated the activities of the new Union, elected its first President and Executive Committee and was readmitted to ICSU.

To preserve its history, the International Mathematical Union maintains an archive containing important correspondence and documents. The archive keeps, in particular, the correspondence of all IMU Prize selection committees. The IMU Executive Committee decided that all material related to IMU Prizes must be kept confidential for 70 years, though. The IMU archive has been at ETH Zürich for a long period of time until it moved to the University of Helsinki in 1994. In the beginning of 2011 the IMU archive was moved to the new permanent IMU Secretariat in Berlin.