Matthias Haarmann pursued his studies in church music at the Musikhochschule in Cologne, where he received piano instruction from Professor Vera von Schnitzler and organ lessons from Professor Jürgen Kursawa. He graduated in 1999 and subsequently became the full-time organist for the parish association of cross Köln-Nord. In this role, he not only served as choirmaster and organist but also took the initiative to organize concerts and workshops.

Since 1994, Haarmann has performed as a soloist in various concerts. Notably, he led a concert series in 2001 in Cologne Bickendorf titled "40 fingers / 424 buttons – five keys," which featured a mix of well-known pieces, including works by G. Merkel and W. A. Mozart, alongside numerous new compositions and arrangements, including pieces by Frank Zappa, all performed by at least two musicians. A highlight from this series was a concert of tangos performed with four musicians.

Haarmann's interests also extend to accompanying silent films, where he emphasizes improvisation across different genres and styles. His repertoire includes classic silent films such as "The Golem: How He Came into the World" (Paul Wegener, 1920), "The Testament of Dr. Mabuse" (Fritz Lang, 1921), "Sunrise: A Song of Two Humans" (Friedrich Wilhelm Murnau, 1927), "La Passion de Jeanne d'Arc" (Carl Theodor Dreyer, 1927), and "Pandora's Box" (Georg Wilhelm Pabst, 1929). He has performed as a soloist on both piano and organ in the Cologne Philharmonic Hall and St. Roch in Cologne Bickendorf, often collaborating with composer Wilfried Kaets.

Haarmann has actively participated in several music festivals in Cologne, contributing as a co-organizer, and he played a vital role as a fellow organizer, coordinator, and director for the "Night of Mysticism" events held in 2010 and 2012 at St. Johannes XXIII Church in Cologne-Chorweiler.

Since 2003, Haarmann has been composing music. His works include a cantata for WANA Lane Publishing titled "God is Calling the Stars," as well as various arrangements of hymns for choir and orchestra. In recent years, he has created several spiritual songs and motets. In 2007, he composed music for a sound installation called "Station of Rest," which combines texts by philosopher Khalil Gibran with soothing minimalist sounds. In 2012, he was commissioned to write three songs based on the Prophet Isaiah for the choir of Hildesheim. Several of his choruses and motets have been published in the choir book "In Praise of God" and the motet book "Praise to God" by Carus-Verlag. In 2014, he was awarded the 1st prize by the Liturgical Institute of Switzerland for his composition "Costs and See," designed for choir or cantor and organ, published by the Liturgical Institute of German-speaking Switzerland.

Between 2003 and 2014, Haarmann also performed as a pianist at the Gospel Academy Krefeld, including a tour in the United States in 2004 with the Gospel Choir Wave of Joy from Bonn, led by Angelika Radones. Additionally, in 2012, Matthias Haarmann took on the role of organist at the archiepiscopal seminary of Cologne.