Gardner was born in Manchester, England and brought up in Ilfracombe, North Devon. His father Alfred Linton Gardner was a local GP and amateur composer who was killed in action in the last months of the First World War. His mother, Emily Muriel Pullein-Thompson, was the sister of Captain Harold J "Cappy" Pullein-Thompson, who was the father of the Pullein-Thompson sisters and their brother, the playwright Dennis Cannan.
Gardner was educated at Eagle House, Wellington College and Exeter College, Oxford. An important figure in his early life was Hubert Foss of Oxford University Press, who published the Intermezzo for Organ in 1936 and introduced him to the composer Arthur Benjamin to whom Gardner dedicated his Rhapsody for Oboe and String Quartet (1935).This work had its first performance at the Wigmore Hall in February 1936. The String Quartet No.1(1938) was broadcast from Paris by the Blech Quartet in 1939, and the anthem The Holy Son of God most High (1938) was also published by OUP. At Oxford Gardner was friendly with Theodor Adorno with whom he played piano duets.
Then came the War. Gardner completed two terms as music master at Repton School, where one of his pupils was the composer John Veale, then a sixth former. In 1940 he enlisted and working first as a Bandmaster and then as a Navigator with Transport Command. It was during the War that ideas for the Symphony No.1 began to form.
"My first symphony assembled itself in my mind in stages during the last year or two of the War. The opening even goes back further to a short piano piece I wrote in 1939 or 1940. At that time I'd no idea that it could be the beginning of a symphony, though I was aware that it hardly constituted a complete piano piece.
Other elements in the score started variously as a mid-war setting of passages from Blake's Book of Thel, a theme I conceived for a set of variations and, in the case of the main theme of the finale, a transformation of the opening of the finale. of my first string quartet which had in fact gained two or three performances in Paris and England by the Blech Quartet in 1939 but with which I was deeply unsatisfied and which I eventually withdrew.
I do not believe it is exceptional for a big work to derive from several sources - there are many examples of such a process in the origin of many of Brahms' best known pieces : the first piano concerto, for example, the German Requiem and the Violin Concerto. In my case it was, of course, due to the fact that I was serving in the R.A.F. around the World and could only conceive music in the scrappiest manner on odd pieces of paper in the most unsympathetic ambiances. Demobilisation, therefore, came as a blessed chance to write at length, which is what I did during the bitter Winter of 1946-7 on those evenings when I did not have to be in attendance at the Royal Opera House, Covent Garden, were I earned my living as a repetiteur. In June 1947 I reached the end of the fair full score, put it aside and began to write an opera that never got performed."
Gardner regarded the end of the War as a new start, set aside his juvenile works and began again from Opus 1. He took a job as a repetiteur at the Royal Opera House, Covent Garden. John Barbirolli discovered the First Symphony (Op2) when Gardner was given the opporunity of playing through his Nativity Opera. According to Gardner this work is "unperformable", which fact was quickly grasped by Barbirolli; however, when Barbirolli asked to see other works, Gardner showed him the Symphony. The first movement needed some re-working because Barbirolli was not convinced it made sense in its original form. The work was scheduled for the 1951 Cheltenham Festival where it caused a minor sensation. Here was a huge, powerful work, brilliantly scored and masterfully structured, by a composer of whom almost no-one had heard!
Many major commissions followed and Gardner was suddenly able to call himself "a composer".