In the early years of her performing career, MacGregor was a prolific composer for the theatre (including work for "Cheek by Jowl", and Oxford Stage Company's production of "Hamlet" at Elsinore Castle and the Edinburgh Festival). She was one of the first artists to be selected for the Young Concert Artists Trust in 1985, and has since performed in more than seventy countries, appearing as a solo artist with many of the world's leading orchestras, including the New York Philharmonic, London Symphony Orchestra, Netherlands Radio Philharmonic Orchestra, Oslo Philharmonic Orchestra, Chicago Symphony Orchestra and Berlin Symphony Orchestra, London Philharmonia, Melbourne Symphony and Sydney Symphony Orchestras, Hong Kong Philharmonic, BBC Symphony and Salzburg Camerata.