Violist Theodor Nüßlein first studied as a junior student at the Hermann Zilcher Conservatory in Würzburg with Max Speermann before continuing his viola studies at the University of Music Karlsruhe with Prof. J.-W. Jahn. His training in Germany was followed by graduate studies as an A. Kade Foundation Scholar at the Juilliard School New York in viola with L. Fuchs and P. Doktor, and in chamber music with H. Shapiro and the Juilliard Quartet.

As a founding member of the Manhattan String Trio, he has performed at the Schleswig-Holstein Festival, Victoria Festival (Canada), Festival "Sondways" in St. Petersburg and many other places such as Beijing, Santiago de Chile, New York. In addition, he was a long-time member of the Bay. Kammerorchester Bad Brückenau and the Münchner Bachsolisten.

His teaching activities began during his studies with a teaching position at the Baden Conservatory in Karlsruhe and led him to the Hermann Zilcher Conservatory in Würzburg and the University of Music Würzburg.

In addition to his teaching activities, Theodor Nüßlein was specialist group spokesperson for strings and vice president of the University of Music Würzburg from 2011 to 2017.