Fassang was born into a family immersed in music. He started his organ studies at the age of 13 with István Baróti and graduated from the Franz Liszt Academy of Music in 1998, where he had studied organ with Ferenc Gergely and István Ruppert, and piano with Ilona Prunyi. The same year he enrolled in the organ class of Olivier Latry and Michel Bouvard at the Conservatoire de Paris. In 1999 Fassang got also accepted to the improvised music department where he studied under Loic Mallié, Philippe Lefebvre, Thierry Escaich and Jean-François Zygel.
As part of his studies he spent a sabbatical year in Japan, where he became the resident organist of the Sapporo Concert Hall. During that time, he performed several concerts in Japan, and recorded his first organ CD on the Kern organ at Sapporo Concert Hall that included music of Bach and Liszt as well as his own improvisations. Since 2002 he has performed concerts in France, Hungary, Germany, Holland, Switzerland, Canada, Japan and in the USA.
In 2003 and 2004 Fassang graduated from the organ class and from the improvisation class of the Conservatoire of Paris with excellent results (first prize with honours). From 2004 to 2008 he taught improvisation at the Musikene (Higher Academy of Music of the Basque Country) in San Sebastián. After 6 years abroad, he decided to return to his native country, and settle down there. Since September 2008, he is assistant professor at the Franz Liszt Academy of Music in Budapest. He was responsible for supervising the construction of the organ of the Béla Bartók National Concert Hall and was appointed artistic consultant of the organ concerts of the Palace of Arts in Budapest in 2006. His improvisational style demonstrates the versatility of the organ through the incorporation of jazz and folk music influences. He continues to play classical works, but he also performs concerts with French saxophonist Vincent Lê-Quang, with Hungarian musician Balázs Dongó Szokolay (flutes, saxophone, bagpipe), Hungarian guitarist Gabor Gado, and Hungarian folksinger, Beata Palya.
Having won several prizes at major international organ competitions since 2002, László Fassang has quickly established himself as a major young talent in the international organ world. He graduated from the Franz Liszt Academy in Budapest and the Paris Conservatoire. In 2000, Fassang took a sabbatical year to become resident organist at the Sapporo Concert Hall (Japan).
Winner of the Gold Medal in Improvisation at the International Organ Competition in Calgary (2002), and winner of the Interpretation Prize in Chartres (2004), Fassang moves as freely and happily in the imagination of great composers as in his own. Open to every musical style, he also enjoys playing with jazz and folk musicians. He gives concerts around the world and taught improvisation at the San Sebastian College of Music. He was responsible for supervising the construction of the organ of the Béla Bartók National Concert Hall (Budapest) and was appointed artistic consultant of the organ concerts of the Palace of Arts (Budapest) in 2006. In the same year he got the Liszt Ferenc Prize awarded by the Hungarian Ministry of Education and Culture and the Prima Prize awarded by the National Association of Entrepreneurs. Since 2008 he holds a teaching position at the Franz Liszt Academy in Budapest.
In the fall of 2014, he succeeded at the National Conservatory of Music in Paris as a teacher of improvisation at one of its masters Philippe Lefebvre, organist of the Notre-Dame Cathedral.