Hopkinson began to play the harpsichord at age seventeen and, during the 1750s, hand-copied arias, songs, and instrumental pieces by many European composers. He is credited as being the first American born composer to commit a composition to paper with his 1759 composition "My Days Have Been So Wondrous Free." By the 1760s he was good enough on the harpsichord to play with professional musicians in concerts. Some of his more notable songs include "The Treaty", "The Battle of the Kegs", and "The New Roof, a song for Federal Mechanics". He also played organ at Philadelphia's Christ Church and composed or edited a number of hymns and psalms including: "A Collection of Psalm Tunes with a few Anthems and Hymns Some of them Entirely New, for the Use of the United Churches of Christ Church and St. Peter's Church in Philadelphia" (1763), "A psalm of thanksgiving, Adapted to the Solemnity of Easter: To be performed on Sunday, the 30th of March, 1766, at Christ Church, Philadelphia" (1766), and "The Psalms of David, with the Ten Commandments, Creed, Lord's Prayer, &c. in Metre" (1767). In the 1780s, Hopkinson modified a glass harmonica to be played with a keyboard and invented the Bellarmonic, an instrument that utilized the tones of metal balls.