Born in Lisbon, Alexandra Mendes began her musical studies at the age of six at the Academia de Santa Cecília. She finished the Curso Complementar de Violino do Conservatório Nacional under the guidance of Alberto Nunes. At 16, with a scholarship from the Calouste Gulbenkian Foundation, she continued her studies with Alberto Lysy at the Menuhin Academy in Gstaad, Switzerland, where she remained for two years. She was a member of the Camerata Lysy, with which she toured in Europe and South America, performing as a violin and viola soloist. She returned to Portugal in 1981 and joined the Gulbenkian Orchestra, where she is currently the leader of the second violins. She has performed solo with the Orquestra da RDP, the Nova Filarmonia Portuguesa and the Gulbenkian Orchestra.

She particularly identifies with the chamber music repertoire, a field in which she has been very active. Since 1986, she has been the First Violin of the Quarteto de Cordas de Lisboa, with which she has performed in many concerts in Portugal and Europe, and also recorded an album with the two Quartets by Joly Braga Santos. She also played on the recording of the full edition of the chamber music works of the composer António Victorino d’Almeida. She has been a teacher at the Lisbon Conservatory since 1997.