Born in London, Behnes was the son of a Hanoverian piano-maker and his English wife. His brother was Henry Behnes, also a sculptor, albeit an inferior one. The family moved to Dublin and there William studied art at the Dublin Academy.
After the family returned to London, Behnes continued his artistic training, studying at the Royal Academy School of Art from 1813, under the tutorship of Peter Francis Chenu.[2] As a painter, he exhibited at the Royal Academy in 1815 and won several medals during the ensuing years. In 1819 he won a Society of Arts gold medal for inventing an instrument to assist sculpture work, having by this time begun to practice successfully as a sculptor.
Despite huge success, being declared "superior to Chantrey" in bust portraiture, he could not manage his finances and was declared bankrupt in 1861. His final work, the statue of Sir Henry Havelock (1861) in Trafalgar Square, has the claim to fame of being the first known statue based purely upon a photograph of his subject.
It is likely that he was an alcoholic and also a gambler, and this was worsened by bankruptcy. He moved to "miserable lodgings" in Charlotte Street.
He was found lying unconscious in a gutter, with only three pence in his pocket, on New Years Day 1864 and died on 3 January in Middlesex Hospital.
He was buried in an unmarked grave in Kensal Green Cemetery. George Cruikshank, who had known him, campaigned to raise money for a monument and to present a bronze bust of Behnes to the National Gallery, but there was little progress and the campaign was abandoned.[4]
In December 2016, a folio of 41 pages of Behnes' sketches (some double-sided) was sold at auction by Cuttlestones Auctioneers & Valuers at Penkridge in Staffordshire, for £2,200.[5][6] It includes sketches of the Duke of Newcastle, Sir Thomas Gresham, and Sir Robert Peel.