Family friend Felix Mendelssohn advised that he study music in Germany, where his teachers included Moritz Hauptmann in Kassel.
There followed three years in Leipzig from 1841, where there was further contact with Mendelssohn and his circle, and where Horsley began composing, including a Symphony in D minor.
A string quartet in C major, the manuscript of which is dated March 1861, was completed shortly after his arrival and is probably the first work for this combination to have been written on Australian soil.
Three weeks after his arrival he was appointed organist of St John's Chapel, New York at a salary of £500 a year, which position he filled to the day of his death.
The orchestral works include his Symphony in D, Op.9 (1842-4), a Piano Concerto (1848), and the overtures Genoveva (1853) and The Merry Wives of Windsor (1857).