Marianne Johanna Adriana van Wessem (born Zaandam, 28 October 1892, died Naarden, 25 September 1980) was the third child of Jan Willem van Wessem and Johanna Maria Laan. She took a bachelor’s degree in literature in Leiden and followed courses at the School for Social Work (School voor Maatschappelijk Werk) in Amsterdam. After World War I she spent several years in Zurich and studied psychology with Carl Gustav Jung (1875-1961), and graphology with Dr Ludwig Klages (1872-1956) and Dr Max Pulver (1889-1952). She took also courses in drawing and painting, and philosophy. When the new Museum of Asiatic Art was created in 1932 in the Amsterdam Stedelijk Museum, she was appointed assistant curator. In 1937 she undertook a study tour on behalf of the Museum, travelled to India, Ceylon (Sri Lanka) and the mainland of Southeast Asia , and arrived in Java in March 1938. In Bali, where she was supposed to stay a few days, she met Walter Spies (1895-1942), the famous painter, musician, photographer and Bali expert, and she eventually settled alternately in Denpasar and Ubud. In 1939 she became secretary of Pita Maha, the association of Balinese painters and sculptors founded in 1936 as an initiative of the prince from Ubud Cokorde Gde Agung Sukawati, Walter Spies, the Dutch painter Rudolph Bonnet (1895-1978), and the Balinese painter and sculptor I Gusti Nyoman Lempad (c. 1862-1978). After having served as assistant curator for a while, Marianne van Wessem succeeded Walter Spies in the course of 1939 as curator of the Bali Museum in Denpasar. She kept this position untill her own internment in January 1943 as a consequence of the Japanese invasion and the Dutch capitulation in 1942. After her release from the camp, and a period spent in Bandung, she resumed her activities in Denpasar untill her departure from Indonesia in the the course of 1947. Her first destination was Los Angeles.

When Walter Spies was interned in 1940 due to his German citizenship, his belongings were confiscated by the Dutch. Through the intermediary of her lawyer M. van Wessem bought the bungalows (Campuan) in Ubud and their content. After her own internment, first in Malang then in Lampersari (Semarang), she could no longer look after Campuan. The bungalows were looted by thieves and the Japanese.

Marianne van Wessem married Dr Wicher Gosen Nicolaas van der Sleen (1886-1964) in 1949 in Brussels. She was his third wife. Trained as a biologist and chemist, Van der Sleen took his PHD degree in Amsterdam in 1912, was teacher of biology, and director of his own laboratory in Haarlem. He made name as a globetrotter, photographer, filmmaker and passionate collector, who extensively published his travel impressions, and deed pioneering research on beads. Van Wessem and Van der Sleen became befriended in Bali in 1939 and travelled together in 1941 over Sumatra. After his release from war camp in Java, Van der Sleen travelled to America where Marianne van Wessem met him in 1947. The couple travelled extensively in North America, Mexico, and in South and Central Africa before they definitely settled in Naarden in 1956. Photographs made by Van der Sleen in Indonesia in 1929 and 1939-1942, and in Mexico in 1948 are also part of the collection Van Wessem. The Museum of the Tropics (Tropenmuseum) in Amsterdam holds hundreds of his photographs and negatives. His collections of artefacts are spread over several musea.