Marjan studied the harp at the conservatories of Tilburg, Maastricht and Arnhem, with Cecille Cooymans, Theresia Rieu and Manja Smits.
She followed masterclasses given by Mariëlle Nordmann, Isabelle Moretti, Germaine Lorenzini and Natalia Shameyva (Bolshoi theatre)
During her study she worked with various professional orchestras in the Netherlands, Germany and Belgium.
After her study Marjan de Haer has specialised in authentic harps, especially the Erard single action and double action harps.
On these instruments she plays with period instrument orchestras such as Anima Eterna, Freiburger Barock Orchester, Akademie für Alte Musik Berlin, Brandenburger Barockorchester, les Agrémens, Concerto Köln, die Kölner Akademie, Collegium Cartusianum and La Goya.
In 2005 Anima Eterna, conducted by Jos van Immerseel, released a CD of Mozart Concertos on which Frank Theuns and Marjan de Haer play the Flute and Harp Concerto. It has received wide critical acclaim (CD of the month in Grammaphon, Goldberg) and many awards including a Diapason d’Or .
On the various harps from her collection she plays concerts with chamber music from the period of the instruments. In the last years she has played C.Debussy, M.Ravel, S.Neukomm, L.E.Jadin, F.Strauss, F.Nadermann,C.Bochsa and others in authentic performances. She also loves to play solo recitals.
Since a few years Marjan is performing baroque music on a Welsh tripel and an Italian tripel harp. She performed the G.F. Haendel concerto and also played Basso Continuo in several concerts and recordings.