| MacLaren Art Centre, Art Classes and Day Camps for Children and Youth | Barrie |
| MacLaren Art Centre | Barrie |
| MADD Canada, Mailing Address, Barrie Simcoe Chapter | Barrie |
| Make-A-Wish, Ontario Chapter | Pickering |
| Making Change, Mailing Address | Barrie |
| Manpower, Barrie | Barrie |
| March of Dimes Canada, Barrie - Bryne Dr, Brain Injury Services - Outreach Activity Groups in Simcoe County | Barrie |
| March of Dimes Canada (GTA) | Toronto |
| Marshall (Dr Peter) Psychologist | Barrie |
| Matthews House, Community and Residential Hospice, Community Support Services, Home Healthcare | New Tecumseth |
| May Court Club of Barrie | Barrie |
| May Court Club of Barrie, Barrie | Barrie |
| May Court Club of Barrie, Barrie, May Court Shop | Barrie |
| McArthur's Berry Farm, Oro Medonte | Oro-Medonte |
| Meals On Wheels, Alliston | New Tecumseth |
| Mediation Centre of Simcoe County | Barrie |
| Métis Nation of Ontario, Barrie Office, Infinity Property Services - Rural Urban Indigenous Housing Program | Barrie |
| Metroland Media | Barrie |
| Midland Midwives By the Bay | Midland |
| Mom Concierge, Mailing address, Services for Birth, Breastfeeding, and Beyond! | Barrie |
| Mom's 4 Mom's Doula Services, No Physical Address | Oro-Medonte |
| Monarchist League Barrie Huronia Branch | |
| Mortgage Centre (The), Christmas Hamper Program | Collingwood |
| MS Canada | Toronto |
| MTM Conservation Association, Tiny Marsh Nature Centre, Tiny Marsh Provincial Wildlife Area | Tiny |
| MultiSport Canada | Barrie |
| Municipal Property Assessment Corporation, Barrie Assessment Office | Barrie |
| Museum on the Boyne | New Tecumseth |
| My Friend's House, Domestic Violence Shelter | Collingwood |
| My Sister's Place | New Tecumseth |