- Main Line Health Corporate Office-Radnor
- Main Line Health Home Care
- MAIN Medical Inc.
- Management Science Associates, Inc.
- Manufacturers Association of Northwest PA
- Mary Beth Scialabba
- Marywood University
- Mascaro Construction Co. L.P.
- Mayview State Hospital
- McCune Foundation
- McGuire Memorial
- McNeil CS and P
- Meade Energy Corp.
- Meadville Medical Center
- MECI PA Racing Services, Inc.
- Medrad, Inc.
- Mellon Financial
- Memorial Hospital
- Mennen Medical
- Mental Health Network
- Merakey Allegheny Valley School
- Merck Research Laboratories
- Mercy Health System
- Mercy Health System of PA
- Mercy Nazareth Hospital
- Mercy Philadelphia Hospital
- Meridia Audience Response
- Messiah Village
- Met-Pro Corp.
- Metro Health Center
- MHF Logistical Solutions Inc.
- Michael Baker Jr. Inc.
- Mid Atlantic Events Magazine
- Millcreek Community Hospital
- Miller Creative Group
- Millers Information Systems
- Mine Safety Appliances Company
- Mongiovi Plumbing and Excavating
- Monongahela Valley Hospital, Inc.
- Morrison Senior Dining
- Moses Taylor Hospital
- MossRehab
- Mothers Working, Inc.
- Mount Nittany Medical Center
- MTF Biologics - Jessup
- Muhlenkamp and Company Inc.
- NAPCO Window Systems, Inc.
- National Association of Black Scuba Divers
- National Association of EMS Educators
- National Demolition Association