Tassos grew up in the Greek town of Livadia, moving back to Athens as a teenager to prepare for his university exams. He attended the University of Athens and, after beginning in engineering, ultimately earned a degree in his intellectual pursuit of choice, mathematics. In 1968 Tassos moved to Paris on a scholarship to study at the Sorbonne, where he obtained a Diplôme d’Études Approfondies and a Doctorat de Troisième Cycle.

Tassos met his future wife, Carmen, in Paris in 1972. They married in Washington, DC, in 1973 and moved to Tampa, Florida, where Tassos earned a PhD in mathematics at the University of South Florida.

A mathematician and later computer scientist, Tassos worked at the Computer Sciences Corporation, US Geological Survey, and National Institute of Standards and Technology (NIST). He worked at NIST for over three decades, focusing on image compression, cryptography, and later quantum information and computation. He published over 50 papers or reports on his research as a sole author and in collaboration with colleagues.

Tassos loved soccer, chess, food and wine, languages, and the art of conversation. He played pickup soccer games at NIST four times a week for the majority of his tenure. His wine cellar had up to 400 bottles at one time, which he loved to share with friends and family over lavish meals and even more intricate argumentation. He had a biting wit and a love for creative problem-solving. Tassos spoke Greek, French, and English fluently and was a devoted student of Spanish and Turkish.

Tassos is survived by his beloved wife of 52 years, Carmen Valenzuela Nakassis. He leaves behind his children Dimitri Nakassis (wife, Sarah James), Constantine Nakassis (wife, Julie Cousin), and Magda Nakassis (husband, Damon Jackman); and grandchildren Carmen Cassandre and Anastasia Eléa Nakassis, and Victor Nakassis and Rafael Nakassis Jackman. He is also survived by his brother Athanasios and his nephew Angelos.