After coming down from Oxford in 1939, 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. 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 when Gardner was given the opportunity 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. Many major commissions followed and Gardner was suddenly able to call himself "a composer". He resigned the job at the Opera House and there followed a remarkable period of creativity. Cantiones Sacrae, Op. 11, Variations on a Waltz of Carl Nielsen, Op. 13 and the ballet Reflection, Op. 14, were all written in 1951 and 1952 and first performed during 1952. He re-wrote A Scots Overture, previously a military band piece, for the 1954 season of Promenade Concerts in 1954. In May 1957 Sadler's Wells put on the opera The Moon and Sixpence, which they had commissioned, and two other major works were premiered that year, the Piano Concerto No. 1 and the Seven Songs, Op. 36 in Birmingham, a work which Gardner wrote as "light relief" while working on the other major works. In 1956 he was invited by Thomas Armstrong to join the staff of the Royal Academy of Music, where he would teach for the best part of thirty years. A few years later he took a part-time job as Director of Music at St Paul's Girls' School, following Gustav Holst and Herbert Howells, and was for a time Director of Music at Morley College. These teaching posts led to the composition of some of his most enduring works, and together with the many holiday courses he worked on as a conductor ensured that he was able to bring practical experience and knowledge to bear on his compositions. Gardner composed prolifically throughout his life, and his works are listed on his website. Among the major works are two more symphonies, two more operas – The Visitors and Tobermory, concertos for Trumpet, Flute, Oboe and Recorder and Bassoon, many cantatas, including The Ballad of the White Horse, Op. 40, Five Hymns in Popular Style, Op. 54, A Burns Sequence, Op. 213, as well as much choral, chamber, organ, brass and orchestral music. Gardner's best known work is the Christmas carol Tomorrow Shall Be My Dancing Day, which was written for St Paul's, as was another popular carol setting, The Holly and the Ivy. His final work was a Bassoon Concerto, Op. 249, written in 2004 for Graham Salvage, the principal bassoonist of the Hallé Orchestra, which was premiered at the Budleigh Salterton Festival in July 2007, by Graham Salvage with the Festival Orchestra conducted by Nicholas Marshall.
Gardner's music, apart from "Tomorrow shall be my dancing day", has been largely unrepresented on commercial records, but in recent years a number of new recordings have been issued, including the 3rd Symphony, Oboe Concerto, Flute Concerto and Petite Suite for Recorder and Strings. In September 2007, Naxos issued his Symphony No. 1, Piano Concerto and the overture Midsummer Ale. David Lloyd-Jones conducted the Royal Scottish National Orchestra with Peter Donohoe as the solo pianist.
