| Abilities In Motion | New Tecumseth |
| Achill Choral Society, Caledon | Caledon (Town of) |
| Ad Standards, Toronto | Toronto |
| Addiction Services Central Ontario, South Simcoe Community RAAM - Alliston, Rapid Access Addiction Medicine Clinic, South Simcoe | New Tecumseth |
| Adjala-Tosorontio, Township of, Municipal Office | New Tecumseth |
| Adjala-Tosorontio, Township of, No physical address, Rest Stations for Extreme Weather Relief | Adjala-Tosorontio |
| Adjala-Tosorontio. Fire Service | New Tecumseth |
| Adoption Council of Ontario | Toronto |
| Advocacy Centre for the Elderly | Toronto |
| Agilec, Innisfil, Integrated Employment Services | Innisfil |
| Al-Anon, Angus / Borden, District 1 - Love and Hope Group | Adjala-Tosorontio |
| Al-Anon, Bradford, District 18 - Journey to Freedom Group | Bradford West Gwillimbury |
| Al-Anon / Alateen, Barrie - No Physical Address | Barrie |
| Alcohol and Gaming Commission of Ontario | Toronto |
| Alcoholics Anonymous, Barrie, Barrie and Area Intergroup | Barrie |
| Alcoholics Anonymous South Simcoe, Alliston, Alcoholics Anonymous | New Tecumseth |
| Alcoholics Anonymous South Simcoe, Borden CFB, Alcoholics Anonymous | Adjala-Tosorontio |
| Alcoholics Anonymous South Simcoe, Bradford, Alcoholics Anonymous | Bradford West Gwillimbury |
| Alcoholics Anonymous South Simcoe, Innisfil - Alcona, Alcoholics Anonymous | Innisfil |
| Alcoholics Anonymous South Simcoe, Innisfil - Stroud, Alcoholics Anonymous | Innisfil |
| Alcoholics Anonymous South Simcoe, Tottenham, Alcoholics Anonymous | New Tecumseth |
| Alectra | Vaughan |
| Alert Ready - Emergency Alert System | Ontario |
| All Saints' Anglican Church. Collingwood | Collingwood |
| Alliston & District Humane Society | Adjala-Tosorontio |
| Alliston Business Improvement Association | New Tecumseth |
| Alliston Diagnostic Centre, Alliston, Medical Diagnostic Imaging and Consultation | New Tecumseth |
| Alliston Family Health Team | New Tecumseth |
| Alliston Food Bank, Alliston | New Tecumseth |
| Alliston Lions Club, Alliston Lions Christmas Food and Toy Hampers | New Tecumseth |
| Alliston Lions Club | New Tecumseth |
| Alliston Lions Club, Loan Cupboard | New Tecumseth |
| Alliston's Attic Thrift Store, Alliston - 39 Victoria St W | New Tecumseth |
| ALS Canada | Hastings County |
| ALS Canada, Mailing Address, ALS Canada Community Services | Barrie |
| Alzheimer Society of Simcoe County, Barrie | Barrie |
| Alzheimer Society of Simcoe County, Barrie, Education Programs | Barrie |
| Alzheimer Society of Simcoe County, Barrie, Social/Recreational/Exercise Programs | Barrie |
| Alzheimer Society of Simcoe County, Barrie, Supportive Counselling | Barrie |
| Alzheimer Society of Simcoe County, Care Partner Support Groups, Barrie | Barrie |
| Am Shalom Synagogue | Barrie |
| Amnesty International, Barrie - No Physical Address, Barrie Amnesty International Action Circle | Barrie |
| Angus and District Lions Club | Essa |
| Angus Cooperative Homes | Essa |
| Angus Food Bank | Angus |
| ARCH Disability Law Centre | Toronto |
| Ask4Care Support Services, Barrie | Barrie |
| Asperger's Society of Ontario | Toronto Downtown North |
| Aspire Community & Developmental Services | Barrie |
| Assaulted Women's Helpline | Toronto |
| Association of Administrative Professionals, Barrie / Simcoe County Branch | Barrie |
| Association of Ontario Doulas, No Physical Address | Toronto |
| Association of Parent Support Groups in Ontario | Aurora |
| Asthma Society of Canada | Toronto Downtown |
| Autism Ontario [Central East] | Toronto |
| Automobile Protection Association, Toronto | Toronto |
| AWARE Simcoe | Simcoe County |