MacCarthy was a talented violinist who had trained at the Royal College of Music and toured with the Boston Symphony Orchestra. From an early age, MacCarthy claimed to experience mystical visions and she maintained an interest in esoteric spirituality throughout her life. In 1905, she accompanied the soon-to-be president of the Theosophical Society, Annie Besant, on a visit to India where MacCarthy was deeply influenced not only by the religious practices of South Asia, but also its music.
This is an interest she further developed with her second husband, the composer John Foulds, whom she married in 1915. They collaborated on his World Requiem and MacCarthy wrote and spoke about Indian music in the UK. She also had an interest in the visual arts and was a founder member of the Theosophical Arts Circle (1907-14) and wrote for their journal, Orpheus. Foulds and MacCarthy met a young man, referred to only as 'The Boy' in her writings, who was employed in a gas works in the East End of London. According to MacCarthy, 'The Boy' possessed great spiritual powers and could channel an initiated spiritual group known as 'The Brothers'. In 1935, MacCarthy, Foulds and 'The Boy' moved to India where they established an ashram to promulgate these spiritual teachings. After Foulds death, MacCarthy took the name Swami Omananda Puri.