| A Cappella Showcase | Waterdown |
| A Child's Voice Foundation, Mississauga | Mississauga |
| A+ Medical Centre and Walk-In | Milton |
| ABC Health Transportation | Milton |
| Ability Online Support Network, Mono - Online Service, Online Support Network | Mono |
| AbleLiving Services, Burlington - Bethany Residence, Supportive Housing | Burlington |
| AbleLiving Services, Burlington - Palmer Place, Supportive Housing | Burlington |
| Aboriginal Legal Services | Toronto |
| AboutFace | North York |
| ABRAR Trauma and Mental Health, ABRAR Trauma and Mental Health Services | Hamilton |
| Access Midwives, Midwifery Services, Access Midwives | Hamilton |
| AccessOAP | Toronto |
| Acclaim Health, Home Care | Oakville |
| Acclaim Health, Hospice Care and Bereavement Support | Oakville |
| Achēv, Oakville, Newcomer Information Centre | Oakville |
| Achẽv, Language Assessment Centres, Mississauga Location | Mississauga |
| Achẽv, Newcomer Information Centre, Mississauga Location | Mississauga |
| Achẽv, Accelerating Career Advancement Program | Mississauga |
| Achēv, Oakville, Language Assessment Centre | Oakville |
| Acoustic Neuroma Association of Canada | Toronto |
| Action Canada for Sexual Health and Rights, Ottawa | Ottawa |
| Active Aldershot Society | Burlington |
| Acton Aqua Ducks Swim Club | Acton |
| Acton Badminton Club, Recreational Badminton | Acton |
| Acton Citizens Band | Acton |
| Acton Curling Club | Acton |
| Acton Foodshare, Food Bank | Acton |
| Acton Medical and Urgent Care | Acton |
| Acton Skating Club | Acton |
| Acton Soccer Club | Acton |
| Acton-Halton Walk In Clinic | Acton |
| ADAPT Halton, Oakville - Head Office, Alcohol, Drug and Gambling Assessment, Prevention and Treatment Services | Oakville |
| ADAPT Halton, Burlington Office | Burlington |
| ADAPT Halton, Community Justice Programs | Halton Region |
| ADAPT Halton, Community Withdrawal Management Services | Halton Region |
| ADAPT Halton, Concurrent Disorders Programs | Halton Region |
| ADAPT Halton, Georgetown Office | Georgetown |
| ADAPT Halton, Milton Office | Milton |
| ADAPT Halton, Problem Gambling & Behavioural Addictions Services | Halton Region |
| ADAPT Halton, Youth Programs | Halton Region |
| Adaptive Clothing, Halton Region | Ontario |
| Adult Learning Centres Halton, Essential Skills for Employment | Georgetown |
| Adult Learning Centres Halton, Acton - Halton Hills Public Library | Acton |
| Adult Learning Centres Halton, Burlington - Centre for Skills Development | Burlington |
| Adult Learning Centres Halton, Get SET | Halton Region |
| Adult Learning Centres Halton, Milton - Centre for Skills Development | Milton |
| Adult Learning Centres Halton, Oakville - Centre for Skills Development | Oakville |
| Adults in Motion, Day Program | Oakville |
| Afghan Association of Ontario, Canada | Toronto |
| African Caribbean Council of Halton | Milton |
| African Community Services of Peel, Brampton, Family Program | Brampton |
| African Community Services of Peel, Brampton, Newcomer Settlement Program | Brampton |
| African Community Services of Peel, Brampton, Violence Against Women Program | Brampton |
| Agriculture Wellness Ontario, Guardian Network | Ontario |
| AgScape | Milton |
| Akwaba Cultural Exchange, Cultural Exchange Program | Milton |
| Al-Anon Family Groups, Halton Hills and Milton, Al-Anon and Alateen Meetings | Halton North |
| Al-Anon Family Groups, Hamilton / Burlington, Al-Anon / Alateen Meetings | Burlington |
| Al-Anon Family Groups, Oakville, Al-Anon / Alateen Meetings | Oakville |
| Alcoholics Anonymous, Oakville - Halton District Office, Halton Area Meetings | Halton Region |
| Alcoholics Anonymous - North Halton/Erin, North Halton / Erin District, Erin and Area Meetings | Erin |
| Alcoholics Anonymous (GTA), Find an Alcoholics Anonymous Meeting - Greater Toronto Area Intergroup | Ontario |
| Alcoholics Anonymous Hamilton, Alcoholics Anonymous Meetings | |
| Aldershot Lions Club | Burlington |
| Aldershot Tennis Club | Burlington |
| Alliance for South Asian AIDS Prevention, Toronto | Toronto |
| Alpha Care Medical Center, Family Practice and Walk-in | Georgetown |
| Anonymous Friend, Youth Peer to Peer Mental Health Support | Oakville |
| Aplastic Anemia and Myelodysplasia Association of Canada | Brighton (Municipality of) |
| Appleby College | Oakville |
| Appleby College, Appleby College Camps and Academies | Oakville |
| Appleby College, March Break Camps | Oakville |
| Appleby College, Newcomers Tutoring Program | Oakville |
| Appleby College, Summer Camp Programs | Oakville |
| Appleby Tennis Club | Burlington |
| Apples and Angels Montessori School and Day Care, Georgetown - 79 Main St, Licensed Child Care Centre | Georgetown |
| ARCH Disability Law Centre, Specialty Community Legal Clinic | Toronto |
| Armenian Family Support Services | Toronto |
| Art Gallery of Burlington | Burlington |
| ArtHouse for Children and Youth | Oakville |
| Artshine / Arts4All, Kitchener, School and Community Arts Programs | Kitchener |
| Arubah Kids Clinic, Pediatric Walk-in/Urgent Care Clinic | Mississauga |
| Asian Community AIDS Services, Toronto | Toronto |
| Assaulted Women's Helpline | |
| Associated Youth Services of Peel, Mississauga | Mississauga |
| Association of Ontario Midwives | Toronto Downtown |
| Asthma Canada | Toronto Downtown |
| Atwell Centre, Oakville, Pregnancy Support Services | Oakville |
| Aura Freedom International, Toronto | Toronto |
| Autism Ontario, Halton Chapter - Mailing Address | Burlington |
| Autism Ontario | Toronto Downtown |
