He was the eldest son of John Slagg, a justice of the peace at Manchester, and his wife Jane née Crighton. John Slagg senior was a commission agent and merchant in the city. His friend, Anti Corn Law campaigner Richard Cobden was his son's godfather.
Slagg followed his father into business, and became president of the Manchester Chamber of Commerce. In 1885 he was appointed as a director of the Suez Canal Company. He made his home at Hopefield, Pendleton.
In 1880 he was elected as one of Manchester's three members of parliament. The Redistribution of Seats Act 1885 divided the Manchester constituency into six divisions, and Slagg stood as the Liberal candidate at Manchester North West, but was defeated. He returned to the Commons at a bi-election in 1887 at Burnley.
- Obituary, The Times, 8 May 1889, p.7
- The New Members of Parliament, The Times, 6 April 1880, p.4
- Richard Cobden, Anthony Howe, Simon Morgan, Gordon Bannerman, The Letters of Richard Cobden: Volume I: 1815-1847, Oxford, 2007
- The Suez Canal, The Times, 1 June 1885, p.5
- Deaths, The Times, 9 May 1889, p.1
- Works by or about John Slagg at Internet Archive
- Hansard 1803–2005: contributions in Parliament by John Slagg
|Parliament of the United Kingdom|
| Member of Parliament for Manchester
1880 – 1885
With: Jacob Bright
Hugh Birley 1880 – 1883
William Henry Houldsworth 1883 – 1885
| Member of Parliament for Burnley
1887 – 1889
Jabez Spencer Balfour
|Professional and academic associations|
| President of the Manchester Statistical Society