William was the son of James and Jane Slack (née Colley).
Both his parents were born in Mansfield, James in 1945 and Jane also in about 1845. They were married at Mansfield St John the Evangelist on 28 October 1866.
There were no details on the 1911 Census of the number of children they had had, but from the names on the census between 1871 and 1911 they probably had at least ten including two who died young. The children were born in Mansfield and baptised at St John the Evangelist: Annie b. abt. 1868 baptised April 1869; Gilbert birth registered 1870 (J/F/M) bap. April 1870 d. 1870; Harry b. 1871 bap. October 1877; Jane b. abt 1874 bap. October 1877; Samuel b. abt. 1875 d. after 1881/before 1891; James birth registered 1877 bap. October 1877; Fred b. 1879 bap. September 1879; William b. 15 March 1881 bap. 7 September 1881; Edith b. 1883 bap. March 1886 and Sam b. 1885 bap. March 1886.
James and Jane were living at 64 Wood Street, Mansfield, when their daughter Annie was baptised in 1869 and still at the same address at the time of the 1871 Census.
The family moved to 3 The Rookery, Mansfield, later that year and was recorded there on the 1881 Census and also in 1891: James, a framwork knitter, Jane, Ann, Harry, and James who were cotton mill hands, Fred, William, Edith and Sam (b. 1885).
By 1901 James, a foundry labourer, and Jane had moved to 8 Westfield Lane, Mansfield. Only four of their eight surviving children were still living at home: Fred an iron moulder, William a stoker (gas works), Edith a hosiery mender and Sam an iron moulder. Three of their children were married: Harry to Agnes Smith Butler in 1892 and living in Mansfield with their son; Jane to Frederick William James in 1897 (Mansfield SS Peter & Paul) and James to Lilian Pears in 1900 (reg. Barrow in Furness) and living in Barrow in Furness where James worked as an iron moulder. The eldest daughter Annie was working as a domestic servant in Sandiacre in the household of Joseph Austin, a lace maker (employer) and his family.
William married Annie Eliza Colman, the daughter of John Keane Colman (grocer), at Mansfield St John the Evangelist on 26 October 1901. They had four children: Annie b. 1902, Ellen Mary birth registered 1904 (J/F/M), William Colman birth registered 1906 (J/F/M) and Sam b. 1907.
The family later emigrated to Canada. William left from Liverpool on 19 November 1909 (SS Empress of Britain) for St John, New Brunswick, while his wife and four children left Liverpool for Quebec on 26 May 1910 (SS Virginian). They were living in Hamilton, Ontario, when William enlisted in 1915 but the later CWGC record gave his widow's address as Forsythe Street, Fort Erie, Ontario.
James Slack snr. died in 1909 and in 1911 his widow was living with her married daughter Edith Gill (m. John William Gill 1905) on Lindley Street, Mansfield. Two of Jane's sons were unmarried: Fred, an asphalter, who was living at 12 Westfield Lane, Mansfield, and Sam who had joined the Royal Navy as a 2nd Class Stoker in October 1906 on a 12 year engagement (5 years RN 7 years Royal Fleet Reserve) and who did not transfer to the RFR until October 1911. Sam married Margaret Matthews in 1913. Only Annie has not yet been traced on the 1911 Census but in 1921 she was living with her married sister, Edith Gill, and her family in Mansfield.
Fred died aged 34 at Lindley Street, Mansfield on 31 December 1913. His mother Jane died the following year on 10 May 1914, also at Lindley Street. Probate was awarded to her son Harry, a chemical works foreman, and her married daughter Edith Gill.
Sam, who had transferred to the Royal Fleet Reserve (Stoker 1st Class) in October 1911, was mobilised on 2 August 1914. He was probably transferred to the Royal Naval Division (Collingwood Battalion) and been interred in Holland after the retreat from Antwerp in 1914. However, he appears to have rejoined the Fleet and was not demobilized until February 1919.