Stefan Thomasz was born in 1944 in Timișoara, Romania. He began his musical training in his hometown and continued at the Bucharest Academy of Music between 1963 and 1968, under the guidance of professors Joseph Prunner and Ion Cheptea. From his very first year of study, he played in recitals and began his soloist career with an orchestra. At his graduation concert, he performed the first part of Antonín Dvořák's Concerto for Cello and Orchestra in B minor Op. 104 from his own transcription, having an orchestral accompaniment for the first time. Later, he presented it in its entirety, in 1974.
He played in double bass concertos accompanied by almost all of the Romanian orchestras, also having recitals in Romania, Hungary and Germany. His discography includes solo works and concerts with orchestral accompaniment, as well as chamber music works. In 1966, he became a member of the "George Enescu" Philharmonic Orchestra in Bucharest and, in 1986, he was appointed as principal double bass player, having worked there until 2008.
Between 1993 and 2021, Stefan Thomasz worked as an associate professor at the double bass and chamber music departments of the Bucharest Academy of Music. Throughout that period, his main focus was to develop educational materials for double bass, as well as to expand the repertoire of the instrument by making various transcriptions and arrangements.