Merrick's peers included Artur Schnabel and Mark Hambourg, and he studied with Theodor Leschetizky. From 1911 to 1929 he taught at the Royal Manchester College of Music and from 1929 at the Royal College of Music. His students included Alan Rawsthorne and Thomas Pitfield He went to Vienna in 1899 for about 18 months and again, for a shorter period, in 1903. This prevented him from playing at the Bechstein (now Wigmore) Hall when it first opened. (). He is particularly known for winning in 1928 the Columbia Gramophone Company competition to write the remaining movements (scherzo and finale) of Schubert's 8th Symphony. His composition was recorded by Columbia and sold, although these recordings are primarily, if not all, on 12-inch records and now very rare. Several later LP recordings of Frank Merrick playing various works by John Field and other British composers were released on the British label Rare Recorded Editions and are also rare. A "Frank Merrick Society" was formed to release his recordings. His book, Practising the Piano, first published by Barrie and Jenkins in 1960, has gone through at least four printings. His many pupils include the composer Michael Garrett.
The original manuscripts of his compositions were donated to CHOMBEC (Centre for the History of Music in Britain, the Empire and the Commonwealth) and the archives of Bristol University. There are many items, including a large number of Esperanto songs, over 30 of which, by his own account, he composed.
During the First World War he was imprisoned as a conscientious objector, and learned Esperanto with the help of fellow-prisoner Henry Montagu Butler. He dedicated much time to the language and spoke it well; in 1965 he made a recording of some of the Esperanto songs with the well-known mezzo-soprano Sybil Michelow.
He spent many hours at the British Museum copying by hand the works of John Field, who at that time was little known; the result was his edition of Field's piano concertos which became vol. 17 of Musica Britannica.
He made several recordings of the music of Sir Arnold Bax, including the composer's first violin sonata (reissued on compact disc in 2003) and the four piano sonatas. Bax's Pæan for piano is dedicated to him. (). With Michael Round he made a recording of the Sonata for two pianos and other works by Bax.
Late in his life he was awarded MMus by Bristol University, when they first introduced the degree. Around 1967 he returned to the Victoria Rooms, Clifton, to celebrate the 75th anniversary of his first public recital, as a child, largely reprising the programme he had played at his début. Even later, in 1976, he visited the London Esperanto Club to be interviewed, in Esperanto, on the occasion of his 90th birthday. He was appointed a CBE in the 1978 New Year Honours.