In 1938, he was one of 16 eminent singers invited to take part in the first performance of the "Serenade to Music", written by Vaughan Williams to mark Sir Henry Wood's Silver Jubilee as a conductor. Apart from the vocal beauty of his singing and overall musicianship, Harold Williams was renowned for his expansive breath control and for the clarity of his diction in English music. Although the concert hall was his natural milieu, he also performed such roles as Iago ("Otello"), Wolfram ("Tannhäuser") and Tonio ("Pagliacci") with the British National Opera Company until its demise in 1929. For 16 seasons, he also sang such parts as Mephistopheles ("Faust") and Boris ("Boris Godunov") at Covent Garden, in London.