At the age of 16 he was organist at St Andrew's Episcopal Church, Milngavie, Dunbartonshire.
He studied piano under Agnes Millar, and organ under John Pullein, Dr. Stanley Marchant, of St. Paul's Cathedral, and Prof. Joseph Cox Bridge of Durham University.
Horner was appointed to the academic staff of the University of Adelaide's Elder Conservatorium in 1927 as a replacement for Harold Wylde, who retired in 1925.
He was one of those (with Frederic Finlay and W. Lawrence Haggitt, W. R. Knox abstaining) who protested the appointment of John Dempster as City Organist in 1929 without due process.
[3] (Harold Wylde succeeded him in 1933) Similar controversies had arisen between W. R. Pybus and T. H. Jones in 1891.