Graduate of the Academy of Music in Katowice, where he completed studies in composition and music theory as well as a PhD course in orchestra and opera conducting under Szymon Bywalec. In addition, he attended Michael Dittrich’s conducting masterclass in Budapest, where he was awarded the 2nd Prize in a competition, as well as masterclasses taught by Gabriel Chmura, Jacek Kaspszyk, and Larry Livingston. He is closely associated with the Archetti Chamber Orchestra, with which he has collaborated as a violinist, and since 2014 as its conductor. In 2015–19 he worked as a conductor of the Polish Youth Symphony Orchestra in Bytom and the Karol Szymanowski Youth Symphony Orchestra in Katowice. 

Tomasiewicz has worked as an assistant to leading conductors including Michail Jurowski, Gabriel Chmura, Jacek Kaspszyk, Michał Klauza, Christoph Koenig, Alexander Liebreich, Tadeusz Kozłowski, and Antoni Wit. He has been assistant conductor to NOSPR Katowice, as well as at the Grand Theatre in Łódź, where (having won a competition for the position of assistant conductor) he prepared the premiere of Moniuszko’s The Haunted Manor directed by Krystyna Janda. He has led orchestras such as Warsaw Philharmonic, Beethoven Academy Orchestra, Polish Sinfonia Iuventus Orchestra, Białystok Opera and Philharmonic, Katowice Philharmonic, philharmonic orchestras in Kalisz, Kielce and Zielona Góra, the Silesian Chamber Orchestra, and Orkiestra Muzyki Nowej (New Music Orchestra). 

Since 2016, he has worked at the Silesian Opera in Bytom, where he has conducted, among others, the balet premiere Sheherezade /Medea to music by Rimsky-Korsakov and Barber, as well as revivals of Puccini’s Tosca, Tchaikovsky’s The Nutcracker, Johann Strauss Jr’s The Gypsy Baron, and Moniuszko’s The Haunted Manor. At the Podlasie Opera and Philharmonic in Białystok his projects have included Bizet’s Carmen and Puccini’s Turandot. In the 2016/17 season, he was assistant conductor to the Polish Orchestra Sinfonia Iuventus, and was twice the Institute of Music and Dance’s conductor-in-residence (at the Katowice Philharmonic and the Białystok Opera and Philharmonic). Since September 2019 (following a competition for the post of Andrey Boreyko’s assistant), he has worked at Warsaw Philharmonic. In the coming season he is taking up the post of principal conductor of the Silesian Chamber Orchestra.