Solicitor, born on 11 July 1857 at Ripley, Derbys. His father John Slack, a successful businessman, WM local preacher and class leader, died in 1882. His sister was Agnes E. Slack.
As a student at Wesley College, Sheffield, 1868-1876, he was a keen sportsman and athlete, and this, with mountaineering, became a life-long enthusiasm. Taking his London BA in 1876, he was articled to a Derby solicitor and after qualifiing in 1880 practised first in Ilkeston. Moving to London in 1890, his friendship with H.P. Hughes drew him into close involvement with the West London Mission, where he served as a circuit steward, sometimes took Hughes' place at the Sunday afternoon meeetings in St. James's Hall and became known as 'the poor man's lawyer'. He was a local preacher and became involved in open-air witness in Hyde Park. He was on the Connexional Local Preachers' Committee from its establishment in 1894 and served on the General Committee of the LPMAA from 1885 and as its President in 1894. He wrote a weekly article for local preachers in the Methodist Times. He was first appointed a representative to Conference in 1886. In 1901 he became the treasurer of the connexional Temperance Committee, and also served on the Home Mission and London Mission committees and on the *Committee of Privileges. In 1891 he accompanied H.P. Hughes to the Ecumenical Methodist Conference in Washington DC and travelled widely in the USA. He was also involved in the work of the YMCA and the National Free Church Council and was a keen Temperancetemperance advocate and opponent of gambling.
In 1904 he won St. Albans from the Conservatives, but lost the seat in 1906. As Liberal MP during that brief period he supported women's suffrage, but his bill was talked out. He was given a knighthood in 1906. The breadth of his other interests is indicated by his membership of the Zoological, Geographical, Palaeographical and Bibliographical Societies. He died at Ripley on 11 February 1909.