After finishing his master degree in Scenography at The Royal Academy of Fine Arts in Ghent and his Master degree in Costume Design at the Royal Academy of Fine Arts of Antwerp, he started his career at Arena Theatre Ghent. He quickly became permanent costume designer and supervisor at the city theatre of Ghent: NTG (now NT Ghent) where he worked with several directors such as Hugo Claus, Jean-Pierre Dedecker, Dirk Tanghe, Eddy Vereycken, Hugo van den Berghe, Guy Cassiers and Victor Löw.

In his last year at NTG, director Frank van Laecke - "The Wizard" - invited him to create the costumes for the opera productions Cavalleria Rusticana and Pagliacci at the National Opera in Amsterdam. It was the start of an intense collaboration.

Since 1997 Marnik Baert is an independent freelancer for various (inter)national productions. In 1999 he started as artistic coordinator of the costume designing department at the Academy of Fine Arts in Antwerp. Nowadays he is professor costume designing at the academy.

For Music Hall Group he was designer for the international arena opera productions  ‘La Bohéme’, ‘Carmen’, ‘Nabucco’, ‘Aida’, and ‘ La Traviata’ which toured throughout Europe, all under the direction of Franck Van Laecke.Marnik Baert designed the stage and costumes for several Music-Hall Group musical productions like ‘Peter Pan’, ‘Alladin’, ‘Camelot’, ‘Oliver Twist, ‘A Christmas Carol’ and ‘Oliver!’.For Festivaria he designed the stages for the musicals 'Kiss me kate', 'Titanic' and will do so for the 2022 production of 'Singing In The Rain'. He was co-director for the musical ‘The Wizard of OZ’.

In 2006 he was asked by The Flemish Opera to create the set for the Nino Rota opera ‘Aladin’, and the following year he was responsible for the set- and costume design for ‘Les Mamelles de Tirésias’ from Francis Poulenc' at the Grand Théâtre Luxembourg.

He designed the sets for the prestigious Historalia musical productions 'Albert I', 'Rubens' and the much anticipated production '1830' in 2021 and 2022.

For Zomeropera Alden Biesen he designed the operas ‘Don Pasquale’, ‘La Traviata’, ‘Die Fledermaus’, ‘Tosca’, 'Il Trovatore', 'L'Italiana in Algeri', ‘Rigoletto’ and ‘Eugen Onegin’.