She became the organist of St Peter upon Cornhill Church in London when she was 14 years old and remained in the post until 1882.
[2] After the death of the prior organist of St Peter upon Cornhill, William Adams, Mounsey competed for the position at 14 years old.
In the autumn of 1840, Gauntlett invited German composer Felix Mendelssohn to St Peter to play the organ, where Mendelssohn played a Johann Sebastian Bach composition and two of his own.
Mendelssohn presented Mounsey with the opening bars of Bach's passacaglia which was then preserved in the vestry of the church.
[5] She was the pupil of German guitarist Ferdinand Pelzer and made her first performance on the guitar on 11 April 1833 at London Tavern.