Born in Münster and raised in Bonn in a family with a musical tradition, he received his first violin lessons from his father at the age of six.
 

Sebastian Casleanu initially received his training as a preliminary study with Zakhar Bron.
 

He then studied with Prof. Rosa Fain at the R.Schumann Hochschule in Düsseldorf and graduated cum laude with Prof. Mihaela Martin at the Hochschule für Musik Cologne off. This was followed by postgraduate studies at the University of Art in Graz with the renowned violin pedagogue Prof. Boris Kuschnir. During this time he was a scholarship holder of the "Studienstiftung des Deutschen Volkes" and the "Deutsche Stiftung Musikleben".

As soloist and leader of the Giocoso String Quartet, Sebastian Casleanu has won prizes at numerous national and international competitions, such as the Jeunesses Musicales Competition, George Enescu Competition, Melbourne Int. Chamber Music Competition and the ARD Competition.

Important solo and chamber music performances (e.g. with Lynn Harrel, Avi Avital, Marc Coppey) document his international concert activities, for example in the Pierre Boulez Saal Berlin, Konzerthaus Berlin, Beethovenhaus Bonn, Musikverein Wien, Konzerthaus Wien, Wigmore Hall London, Aix- en Provence Festival-France, Beethovenfest Bonn, Eilat
Festival - Israel, National Grand Theater Beijing, Concert Hall Shanghai, Idyllwild Arts Festival - Los Angeles, Seoul Arts Center, Melbourne Recital Center and Sydney Recital Hall.
 

There are numerous TV and radio recordings in Europe, Australia and Korea. Sebastian Casleanu has already given concerts for political personalities such as: the former Foreign Minister Guido Westerwelle, the former Federal President Johannes Rau, the former Foreign Minister Hans Dietrich Genscher, for the Ambassadors of Romania, Bulgaria, the Foreign Minister and the Vice President of the People's Republic of China.

From 2016 - 2017 he was 1st concertmaster of the Rhineland-Palatinate State Philharmonic and since 2019 he has been 1st concertmaster of the Nuremberg State Philharmonic.