Marie Thornam came from a family, in which many of the women chose an artistic education - she was the cousin of Emmy and Ludovica Thornam, who both made a living as painters - respectively of landscapes and flowers and of portraits and genre-paintings.

Marie Thornam was not accepted at the academy, but was taught by her father whom she later assisted in the colouring of drawings for Flora Danica and for illustrations for Botanisk Tidsskrift (The Botanical Journal). Following her drawing education, she was taught by the landscape painers Hans Fischer (1849–1886) and Andreas Fritz (1828–1906). She found the motifs for her landscapes in the area around Lyngby, Sorgenfri and Bagsværd in Northern Zealand and from 1887 she exhibited several times at Charlottenborg.

Marie Thornam managed - as only very few women of her time - to make a living as an artist and a teacher at the school of drawing and painting, which she ran in the early 1890'es in Lyngby.