William was a son of Thomas Magarey"s brother James Magarey (ca1818 – 11 August 1859), who ran Gannawarra Station on Gunbower Creek (a tributary of the River Murray), later owned a flour mill in Hindmarsh, South Australia, then moved to "Laurel Bank Villa", Geelong, Victoria and drowned following the wreck of the Steamship Admella.
William worked on his father"s station and moved to Geelong with him. He took over the flour mill at Hindmarsh and purchased one at Portuguese Pirie. He sat in the House of Assembly seat of West Torrens from April 1878 to March 1881.
He was on the boards of the Savings Bank, National Mutual Life Association, and the South Australian Woollen Company.
He was also Chairman for some years of the Executor and Trustee Agency Company and British Broken Hill Proprietary. Around 1908 he became afflicted with rheumatism, but refused to relinquish any of his duties until incapable of walking.
He was an enthusiastic member of South Australia"s Volunteer Force (a pre-Federation militia).
