Wilhelmj was considered a child prodigy. When Henriette Sontag heard him in 1852, when he was seven, she said "You will be the German Paganini". In 1861, Franz Liszt heard him and sent him to Ferdinand David with a letter containing the words "Let me present you the future Paganini!".
He has become famous for his late nineteenth century arrangement of the second movement of J. S. Bach's Orchestral Suite No. 3 for violin and piano, known as Air on the G String and for his re-orchestration of the 1st movement of Niccolò Paganini's Violin Concerto No.1 in D major Op.6 (1883/84). From 1894 on he was a Professor of violin at the Guildhall School of Music and Drama.
Among his pupils were American violinist Nahan Franko, Canadian musician Donald Heins, and the Australian conductor Aylmer Buesst. Wilhelmj owned a Stradivarius 1725 violin from 1866 until his retirement, which later came to be known by his name.