Lou Koster received lessons in music theory, violin and piano from her grandfather Franz Ferdinand Bernhard Hoebich, the first conductor of the Luxembourg military band. According to her own statement, she already felt called to be a composer as a child. At that time, however, the subject of composition was not yet being taught in Luxembourg and studying abroad was not possible for financial reasons. Instead, she took lessons in harmony at the conservatory, which was founded in 1906, and continued her education as an autodidact. Her first works were French songs, but she soon ventured into more extensive compositions (stage and orchestral works as well as orchestral songs). In addition to her work as a composer, Koster has appeared as a pianist, violinist, singer, silent film accompanist and orchestra conductor and worked for forty-six years as a piano teacher at the Luxembourg Conservatory.

As one of the first female composers in Luxembourg, she was dealing with prejudices against female composing in her early and middle years more than her colleagues abroad and suffered greatly from them. Her most demanding compositions could not be inscribed in the musical repertoire. It was only with « Der Geiger von Echternach », which was deliberately composed for a wider audience, that shortly before her death she succeeded in being celebrated by the public as a « national composer ».

Her works were privately owned for a long time and only became accessible to the public in 2003 in the Lou Koster archive of Cid-femmes. The Luxembourg National Library recently acquired an extensive collection of manuscripts. Nevertheless, almost a quarter of the oeuvre remains lost or is only preserved in fragments. Around two hundred and fifty works (songs, piano, chamber and choral music, orchestral works and songs, works for choir and orchestra, a secular oratorio, an operetta, incidental music) have survived in complete versions.