Following the completion of a Bachelor of Arts degree in history from Portland State University in 1962, Allsen attended the University of Washington to pursue a Master of Arts in Russian studies.
In 1969, he graduated from the University of Oregon with a Master of Library Science degree.
The next year, Allsen joined the faculty of the Trenton State College History Department.
[2] Between 1986 and 2013, Allsen served on the editorial staff of the journal Archivum Eurasiae Medii Aevi.
The journal published a Festschrift for Thomas T. Allsen in Celebration of His 75th Birthday in its 21st volume (2014–15).