In 1965, Dannemann participated in the German Figure Skating Championships representing the club Düsseldorfer EG. She came in 16th position.
Dannemann was first introduced to Jimi Hendrix on 12 January 1969, in Düsseldorf, after being invited to a Jimi Hendrix concert there. She spent that night with him and part of the next day too, when she accompanied him to his next concert in Cologne; after that, she returned to Düsseldorf. He spent the last night of the tour with famous model and "scenester" Uschi Obermaier, with whom he was filmed kissing and petting outside the Kempinski Hotel the next morning. He wrote to Dannemann on 25 March 1969, inviting her to visit him in New York City. Dannemann claims she next saw Hendrix when she travelled to London on 25 April 1969 in the hope of meeting him again, where she bumped into him at the Speakeasy Club. She says they spent some time together over the next nine days, but she only spent one night with him.
During this last visit, Dannemann claimed that Hendrix had asked her to marry him, and she said she would have done so if he had not died. Dannemann is known for being the last person to have seen Hendrix alive. On the evening of 17 September Hendrix took at least one amphetamine pill (known as a "black bomber") at a party, where he stayed for a short while. Later, at Dannemann's flat, Hendrix took nine of her Vesparax sleeping tablets; the recommended dose was 1/2 to 1 tablet. Dannemann stated that on the morning of 18 September 1970, she found Hendrix in a coma at her basement flat. She called for an ambulance which arrived at 11:27 AM. Hendrix was officially declared to have died at St. Mary Abotts hospital at 12:45 PM. The cause of death was asphyxiation through aspiration of vomit due to a barbiturate overdose.
After his death, Dannemann held onto Hendrix's famous black Stratocaster (nicknamed Black Beauty by Hendrix). It was kept in its case until 1993, when it was examined by Len Jones. The guitar was last seen in public at a guitar event on 3 December 1995 in London.
In 1996, Dannemann was found guilty of contempt of court for breaching an order of the High Court of England and Wales not to repeat false allegations she had previously made against Kathy Etchingham. Two days later Dannemann, aged 50, was found dead in a fume-filled Mercedes Benz near her cottage in Seaford, East Sussex. Her death was ruled a suicide, but Roth publicly stated his opinion that her death had been the result of foul play. Roth dedicated later works to the memory of Dannemann.