The son of Aubrey Brain, for many years principal horn of the BBC Symphony Orchestra and a prominent figure on the London musical scene, the young Dennis began his career playing second to his father.
Brain first recording was of Mozart's Divertimento no. 17 with the Busch Quartet and the Brains, father and son. Aubrey Brain played first horn in the Busch Chamber players who made recordings in the late 30s and Dennis may have appeared in some of these as second horn. But his solo career really begins with the first performance and recording of Britten's Serenade for Tenor, Horn and Strings, which was written for him and Peter Pears, and in which he was able to display his range of technique and expression. At this time, and at the age of 21, he was appointed first horn of Sidney Beer's National Symphony Orchestra, and his distinctive tone can be heard on their recordings, such as Falla's El Amor Brujo, Elgar's In the South and Wagner's Siegfried's Rhine Journey. Drafted into the Royal Air Force, he continued to establish his position with a famous recording of Beethoven's Horn Sonata with the pianist Denis Matthews.
After the war Dennis Brain was invited to lead the horn section of two new orchestras, Walter Legge's Philharmonia and Sir Thomas Beecham's Royal Philharmonic. This was possible because these orchestras, like many London orchestras in the past, were formed from a pool of available players for each concert. Dennis Brain would not therefore have appeared at every concert by both orchestras, but he is usually present in their recordings, as it was invariably the aim to field the "first team" in the studio. In April 1954, however, Brain found the tenure of both jobs impossible and left the RPO.
He can be heard in the Philharmonia recordings of the Siegfried Idyll and Brahms' First Symphony with Guido Cantelli, Tristan and Isolde with Wilhelm Furtwangler, the Brahms symphonies with Toscanini and Klemperer, Sibelius symphonies with Karajan, and many other recordings made up to August 1957. In the RPO one of his most enduring recordings is in Delius' Mass of Life. He also made several solo recordings, including both Richard Strauss concertos and all four Mozart Concertos.
Dennis Brain was killed in a road accident while driving back from the Edinburgh Festival in his Triumph Sports Car, in August 1957. He loved cars and sometimes had a car magazine on his music stand. Part of his final recital at the festival has survived on disc. The following day he was due to play in a recording of Strauss' opera Capriccio and to this day some people still erroneously believe that he can be heard in this recording, which was conducted by Wolfgang Sawallisch. Francis Poulenc's Elegy for horn and piano was composed as a memorial.