Mary O’Brien was born in Dublin, Ireland. She studied at the Paris Conservatoire for five years with Gabriel Bonillon on a French Government Scholarship. Later she taught the violin at Bedales School, Petersfield. In Italy she attended Venice Summer Schools, having lessons with the Virtuosi di Roma and Quartetto Italiano. For two years she toured throughout Italy as a member of the Santa Cecilia String Orchestra.
On Return to England in 1959 she joined the Halle Orchestra under John Barbirolli. A period with the BBC Northern Orchestra was later followed by a long period as a member of the prestigious Philharmonia Orchestra which was regularly conducted by Otto Klemperer, Carlo Maria Cuilini, Lorin Maazel, Sir Adrian Boult and Daniel Barenboim.
At a competition in Cardiff, 1966 Yehudi Menuhin had described Mary as an,
“excellent violinist; projecting personality; sensitive; and an excellent performer”
In 1976 Mary accepted the position of Senior Lecturer in Violin at the University of Auckland. She regularly broadcasts, has toured with the Federation of Chamber Music Societies and gives concerts with orchestras and chamber music groups.