Born in Poland in 1990 into a family of musicians, he began his musical studies with his father. After high school, he entered the Poznań Music Academy where he studied organ with Elżbieta Karolak and Jarosław Tarnawski. In 2011 he entered the Conservatoire National Supérieur de Musique et de Danse de Paris as a pupil of Olivier Latry and Michel Bouvard. Since 2013, Mossakowski has also been pursuing studies in improvisation class with Thierry Escaich, Philippe Lefebvre and László Fassang.
He is a laureate of several international organ competitions: He won the 1st Prize in the IVth Feliks Nowowiejski International Organ Competition in Poznań in 2010, the 1st Prize in Prague Spring International Organ Competition in 2013, the 1st Prize and Public Prize in the Grand Prix Jean-Louis Florentz Competition in Angers in 2015 and the 1st Prize and Public Prize in the Grand Prix André Marchal Improvisation Competition in Biarritz in 2015. In 2016 he won one of the the most prestigious organ improvisation competition « Grand Prix de Chartres » In 2012 he was the recipient of a scholarship from the french government. Later on, he received the scholarship from the Fondation de France.
In 2014-15 Mossakowski was appointed Young Artist in Residence at Cathedral of St. Louis King of France in New Orleans (USA). There, he played many solo recitals and concerts, including with the Louisiana Philharmonic Orchestra. During 6 months, he used his pedagogical skills by teaching interpretation and improvisation lessons, and gave masterclasses to the public of the American Guild of Organists.
In the 2016-2017 season he twice appeared at the Concert Hall of the Mariinsky Theatre (with Sofia Kiprskaya; in February and during the Stars of the White Nights festival in June), and has performed concerts at the Warsaw Philharmonic (with trumpeter Sergei Nakariakov), the Béla Bartók National Concert Hall in Budapest (with organist Martin Sturm) and the Cathédrale de Nancy. Engagements for the current season include appearances at Notre-Dame de Paris, the Maison de la Radio France and Hesston Mennonite Church (Kansas, USA).