William M[acGregor] [H] Mathers married Mary Ann Collins in 1848 [Q4, St Pancras, Vol 1, Page 302].
According to Samuel's birth Certificate [8 Jan 1854], William was a Merchant's Clerk residing at 11 De Beauvoir Place, West Hackney, London, England (since renumbered 108 De Beauvoir Place, London N1: it still exists, with a green front door!).
The 1861 census shows Mary A Mathers as Head/Widow of the household, living with Samuel L Mathers, Son/Scholar; so William must have died between 1854 and 1861.
Presumably you have read Ithell Colquhoun's biography of S L [M] Mathers, "The Sword of Wisdom".