Sir William is the 27th hereditary chief of the Clan Macpherson of Cluny (Cluny-Macpherson). He was educated at Summer Fields, Oxford; Wellington College, Berkshire; and Trinity College, Oxford. During the period 1944-1947, Sir William served in the Scots Guards, transferring to the 21st Special Air Service Regiment (Territorial Army) with whom he served until 1965. He has served as the Honorary Colonel of the 21st SAS since 1983.
In 1962 Sir William married Sheila McDonald Brodie. They have three children: Annie, the late Alan Thomas (younger of Cluny) and James Brodie (Tanistair of the House of Cluny-Macpherson). The family's home is Newton Castle, Blairgowrie, Perthshire. Blairgowrie has been the home of the Macphersons since 1787, when it was purchased by James "Ossian" Macpherson as the agent for Cluny's ancestor.
Sir William was a Judge of the High Court of England and Wales (Queen's Bench Division) where he served as Presiding Judge of the Northern Circuit until his retirement in April 1996. Before his appointment as a judge, Sir William was a Queen's Counsel practising in London and abroad.