Born Mary Postans in Kent, daughter of Thomas—the Esq. of Tewkesbury—and Agathe Postans and sister-in-law of the writer Marianne Postans, author of "Western India", Shaw entered the Royal Academy of Music in London in 1828. She studied there through 1831 and was a singing pupil of George Smart at the school. She made her professional singing debut in 1834 as a concert singer, and was highly active as a concert and oratorio singer in her native country for the next five years. In 1835 she married the painter Alfred Shaw and thereafter appeared under the name Mary Shaw. That same year she performed at the Concerts of Ancient Music in London and at the York Festival. In 1836 she sang at the festivals in Norwich and Liverpool, the latter in the English premiere of Felix Mendelssohn's oratorio St. Paul on 3 October 1836. In 1837 she appeared in several concerts in London sponsored by the Royal Philharmonic Society and the Sacred Harmonic Society. She also performed at the Birmingham Triennial Music Festival. In 1838–39 she appeared as a soloist with the Leipzig Gewandhaus Orchestra in 12 concerts under Mendelssohn's baton at the Gewandhaus in Leipzig, Germany.
