- McGuffy Industries Inc.
- McLean Cargo Specialists, Inc.
- McLennan County Medical Education and Research Foundation
- MDS Research Company, Inc.
- Med-Loz Lease Service, Inc.
- Medical City Arlington
- Medical City Fort Worth
- Medsource
- MedTrials, Inc.
- Memorial Hermann Greater Heights Hospital
- Memorial Hermann Health System
- Memorial Hermann The Woodlands Hospital
- Memorial Hermann-Texas Medical Center
- Methodist Ambulatory Surgery Hospital - Medical Center
- Methodist Children's Hospital
- Methodist Healthcare
- Methodist Heart Hospital
- Methodist Hospital
- Methodist Hospital Northeast
- Methodist Hospital South
- Methodist Hospitals of Dallas
- Methodist Hospitals of Dallas - Data Coordination
- Methodist Specialty and Transplant Hospital
- Methodist Stone Oak Hospital
- Methodist Texsan Hospital
- Metropolitan Methodist Hospital
- Metropolitan Transit Authority METRO)
- Michael E. DeBakey Veterans Affairs Medical Center
- MICROMAIN Corporation
- Midcoast Energy Resources Inc., Transportation Group
- Milam and Company Painting, Inc.
- Mir, Fox and Rodriguez PC
- Miramar Designs Ltd.
- Mission Regional Medical Center
- Mission Trail Baptist Hospital
- Mitratech
- Montgomery County Youth Services
- Morningside Ministries at Chandler Estate
- Morningside Ministries at Menger Springs
- Morningside Ministries at the Manor
- Morningside Ministries at the Meadow
- Morningside Ministries Senior Living Communities
- Motel 6 Van Horn
- MoveN Software, Inc.
- MPI-The Meeting Professional Magazine
- MRI Technologies
- Multigen. Paradigm
- Muniz Engineering Inc.
- Mustang
- Nabors Industries Inc.