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Warning: This is not intended to be an exhaustive history of the companies listed, but an indication of the changes of ownership and the links between companies and directors in those companies. Only collieries, pits etc. in the North of England are shown - the companies may own other properties etc. outside this area and there may have been other changes of ownership etc. for which we currently do not have details.
Entries added or updated since 01 Aug 2008 are highlighted thus: 
| Company | Company Details | |
|---|---|---|
| Mandall's Slate Co. Ltd. | ||
| Marple & Gillott Ltd. | ||
| Martrac Mining Ltd. | ||
| McGhie, Thomas & Sons Ltd. | ||
| McKechnie Brothers Ltd. | ||
| McLauchlan, Joseph Ltd. | 1923 | |
| Melkridge Coal Co. Ltd. | 1940 | |
| Mickley Coal Co. | ||
| Mickley Coal Co. Ltd. | 1923, 1933, 1940, 1947 | |
| Mid Durham Coal Co. Ltd. | 1933, 1940, 1947 | |
| Middlestone Moor Coal Co. Ltd. | ||
| Midgeholme Colliery Co. | 1933 | |
| Millfield Grange Colliery Co. Ltd. | ||
| Million, A. | 1933 | |
| Millom & Askam Hematite Iron Co. Ltd. | ||
| Millom & Askam Iron Co. Ltd. | ||
| Moresby Coal Co. Ltd. | 1923, 1933, 1940, 1947 | |
| Morley Colliery Co. Ltd. | ||
| Morrell, Watson | 1933 | |
| Morris Collieries Ltd. | ||
| Morwood Colliery Co. Ltd. | 1947 | |
| Mrs. E. L. Walton | 1933 | |
| Mrs. J. T. Walker | 1933 |
