Marijnen studied directing at the Royal Institute for Theatre, Cinema & Sound in Brussels. He began directing for the Mechels Miniatuur Teater while a student at RITCS, and in 1966 directed a production of Edward Albee's The Zoo Story that received positive reviews. In 1966, Marijnen met Polish theatre director and theorist Jerzy Grotowski at a workshop in Brussels. In 1967, Marijnen went to Poland for an internship at Grotowski's Laboratory Theatre in Opole. He was inspired by Grotowski's method, which focused on the actor's physical presence on the stage. Marijnen wrote and published a report about Grotowski's workshop in the theatre magazine Windroos. The report was later reprinted in Grotowski's 1968 book Naar eenarm theater. Marijnen tried to apply Grotowski's method when he returned to Belgium in 1969. He taught workshops at multiple Flemish and Dutch theatre companies, including Dutch Comedy in Amsterdam, and found that classically trained actors were not open to the method. In 1971, Marijnen moved to the United States, looking for a more receptive environment to teach and apply Grotowski's method. He had several teaching appointments while in the United States, including at the Carnegie Mellon School of Drama. In 1973, he founded his own theater company, Camera Obscura, which was based in Jamestown, New York. He directed multiple productions at La MaMa Experimental Theatre Club in New York City during the early 1970s, including Fernando Arrabal's Fando and Lis, Camera Obscura in Andy Wolk's Oracles, Camera Obscura in Wolk's adaptation of Comte de Lautreamont's Maldoror, and Camera Obscura in Shakespeare's Measure for Measure. Camera Obscura went on tour in Europe in 1973, 1974, and 1975. He returned to Europe and, from 1977 to 1983, was the first director of the Ro Theater in Rotterdam. At the Ro Theater and as a freelance director at other theaters, he produced large-scale productions such as Wasteland and Bataille Bataille. He continued using Grotowski's method, and was one of the few directors who successfully produced large-scale productions that were experimental. In 1993, Marijnen became the director of the Royal Flemish Theater in Brussels. He produced of classical pieces such as King Lear and Oedipus / In Kolonos. He brought audiences from French-speaking Brussels and endeavored to give Arab culture a place in the theatre. He resigned in 2000 after seven years as director, partially due to financial issues the theatre was experiencing. He then joined the National Theater in The Hague, where he directed productions including Glenn Gould and Pier Paolo Pasolini - PPP. Since 2012, he has again been directing in Mechelen at Arsenaal, the former Mechels Miniatuur Teater, where he started his career. In early 2014, he produced Scarlatti with an international cast.
