Thomassen holds BA and MA degrees in Anthropology from the Institute of Anthropology at the University of Copenhagen (1994 and 1997) and a Doctoral Degree (Ph.D.) in Political and Social Science from the Department of Political and Social Sciences (2001), at the European University Institute, Florence, Italy.
He wrote his PhD thesis as a narrative evolving from the life-stories of self-identified Italians from Istria who for various reasons chose to leave Tito's Yugoslavia and moved “back” to Italy.
In an article from 2012, “From Liminal to Limivoid: Understanding contemporary Bungee Jumping in a cross-cultural perspective”, co-authored with his cross-cousin Maja Balle, he coined the term “limivoid” to denote liminal experiences that are essentially void of experiential substance and transformative potential.
Elaborating Victor Turner's notion of the “liminoid”, he argued that the “limovoid” critically identifies an underlying aspect of the larger (post)modern celebration of boundary experiences.
With reference to Italy, he discusses how the past is continuously but variously used in the articulation of people's urban, regional, national and European identities.
[9][10] Together with former student, Derrick Fiedler, Thomassen has revisited the work of Arnold Toynbee and argued for its contemporary relevance.
In an article from 2011, co-authored with Arpad Szakolczai, he argued that Gabriel Tarde should be recognized as a founding figure of political anthropology.