Lassen (born August 27, 1901, in Lyngby, Denmark) studied under dancer Valborg Borchsenius at the Royal Theater from 1909 to 1918. The ballerina danced at the Royal Theater from 1918 to 1921 before relocating to the United States to study with Michel Fokine from 1921 to 1924. Returning to Denmark, Lassen scored a triumph as "Swanilda" in Coppelia. Her reputation as a strong technician was further assured by technically flawless performances as the "Ballerina" in Fokine's 1925 production of Petrouchka, and as the first Danish Firebird in 1928. In 1930, stage producer-director Max Reinhardt was so impressed with Lassen during a visit to Denmark that he created a waltz for her in his production of Strauss' Die Fledermaus. She performed the piece in Berlin on May 30, 1930, Reinhardt's 25th professional anniversary. On September 19, 1930, the 26-year-old premier danseuse of the royal Danish ballet was discovered dead in the bed of her Copenhagen apartment from a self-inflicted pistol wound to the heart. According to press reports, Lassen suffered from diabetes, but was also depressed over her rocky marriage to a medical doctor. Lassen's body was found by the man after he reportedly returned from a night out spent in the company of a Danish countess.