Sir William, married with four daughters, lives at Elton Hall, near Peterborough, the home of the Proby family since 1660. The family continues to maintain and preserve their home and welcome visitors to the Hall.
Educated at Eton College, Sir William graduated from Lincoln College, Oxford with a 2(1) in Engineering and Economics and is a Master of Arts of Oxford University.
Sir William was President of the Historic Houses Associaton from 1993 to 1998. He was a member of the Advisory Council of Tate Britain, and was appointed a Fellow of the Institute of Chartered Accountants (FCA).
In 2002, Sir William succeeded as the 3rd Baronet Proby, of Elton Hall. He was Chairman of the National Trust from 2003 to 2008. He was Chairman of the Trustees of the National Portrait Gallery from 2011 to 2017.
Sir William was awarded a CBE in 2009 for services to conservation and heritage, and was the High Sheriff of Cambridgeshire for 2001/2. He was appointed a Deputy Lieutenant of Cambridgeshire in 1995 and retired in 2024.