He was born in Sarvar (Hungary) in 1960. He learnt to play the piano at a very early age, and followed this with the trombone studies. In 1978 he graduated from the Hungarian Army's Higher School of Music, where he specialised in piano and trombone. He continued his studies at the Frank Liszt Conservatory of Music, where he obtained a Distinction in Orchestra Conducting in 1982.

While pursuing his studies at the Conservatory of Music, Marosi conducted the band of his old Higher School of Music (1978-1984). After completing his degree, he was appointed conductor of the Central Hungarian Army Band (1982-1995), and was later appointed Artistic Director of Military Music. During that time he was also guest conductor of the Military Academy's Symphonic Orchestra (1984-1994) and regular conductor at the Operetta Theatre in Budapest (1990-1993).

He has been teaching orchestra conducting at the Frank Liszt Teacher Training School and Budapest Symphonic Band, where he is still working to promote new Hungarian composers, since 1985. Since 1982, Marosi has recorded most of Hungary's contemporary band music on Hungarian radio. He has also been invited to conduct and give conferences in various countries including the United Kingdom, Germany, the United States, Austria, Israel, Italy, the Netherlands and Switzerland. He has been a researcher at the International Society for the Research and Promotion of Wind Band Music (IGEB) since 1992. In 1994, he co-founded WASBE Hungary, where he still works as artistic director. Laszlo Marosi has been a member of the WASBE International Council since 1995.