Fétis sent him to Adolf Friedrich Hesse in Germany to learn Johann Sebastian Bach's tradition.
One year later he published his first work for organ: Dix improvisations dans le style sévère et chantant ("Ten improvisations in a strict and singing style").
In March 1849 he was appointed organ teacher at the Royal Brussels Conservatoire, aged only 26; and he trained numerous young musicians, including two eminent Frenchmen, Alexandre Guilmant and Charles-Marie Widor.
At the request of the Belgian clerical authorities, Lemmens returned to Belgium to found a school for Church music in Mechelen in 1879.
Lemmens could only work at the development of this school for two years, as he died on 30 January 1881.