In 2000 and 2001, he participated in the Kammeroper Schloss Rheinsberg Summer Opera Festival. In the 2004/2005 season, he was a member of the Staatstheater Nürnberg ensemble, and in the 2005/2006 season, as the first Pole, he joined the Atelier Liryque Opéra National de Paris. Finalist and laureate of special awards at competitions for vocalists in Duszniki-Zdrój (2000) and Belvedere in Vienna (2007).
He performed under the baton of numerous renowned conductors, including Seji Ozawa, Kent Nagano, Dennis Russel Davis, Thomas Hengelbrock, Sylvain Cambeling, Hartmut Haenchen, José Maria Florêncio, Tadeusz Kozłowski, Bernhard Kontarsky, Guillaume Tourniaire, Philippe Augin, Wojciech Michniewski, Piotr Wajrak, Tomasz Tokarczyk, Andriy Yurkevych, Ewa Michnik and Gabriel Chmura; he also worked with directors such as Robert Carsen, Robert Wilson, Luc Bondi, Steven Taylor, Karl Ernst i Ursel Hermanns, Hinrich Horstkotte, Krystyna Janda, Krzysztof Warlikowski, Michał Znaniecki, Dominique Pitoiset, Pet Halmen, Marc Paquien, Andre Engel, Marek Weiss and Gilbert Deflo.
He has performed in theatres and on concert stages in a number of countries, including Poland, Germany, Hungary, Austria, France, the United Kingdom, Norway, the Netherlands, Belgium, Canada and China.
In 2008, he took part in the concert performance of Richard Wagner's Tannhäuser (as Wolfram von Eschenbach) in Montreal with the Montreal Symphony Orchestra under the baton of Kent Nagano.In the field of contemporary music, he performed works by Luca Lombardi (including Un oratorio materialistico) and Siegfried Matthus at the 21st century Music Festival in Saarbrücken (2002). He took part in the world première of Xavier Dayer's opera Les Aveugles based on Maeterlinck's drama at Théâtre Gérard Philipe de Saint-Denis in Paris, and then in Opéra Bastille and the London Almeida Theatre (2006). In the 2009/10 season, Garnier also performed Le Forcheron’s part in Philippe Fénelon's contemporary version of Faust under the baton of Bernhard Kontarsky at the Paris Opera.
He is also appreciated as an interpreter of songs, especially German vocal lyricism – compositions by Schubert, Schumann, Brahms, Mahler, Wolf. In 2008, together with the Paris Opera Orchestra led by Vello Pahn, he performed the Lieder eines fahrenden Gesellen by Mahler, a reminder of the most outstanding choreographies by Maurice Béjart.
Together with the ensemble of the Baltic Opera in Gdańsk, he recorded Elżbieta Sikora's opera Madame Curie. Its première took place in 2012 at the UNESCO headquarters in Paris.