More than 118 major companies with 100 or more employees are headquartered in Montgomery County.
| Company | Industry | Primary City |
| A. Morton Thomas and Associates, Inc. | Engineering | Rockville |
| ABT Associates | Government Contractor | Bethesda |
| Access Intelligence LLC | Manufacturing | Rockville |
| Adventist Healthcare, Inc. | Healthcare | Rockville |
| Aeras | Healthcare, Pharma & Biotech | Rockville |
| American Capital Strategies, Inc. | Finance | Bethesda |
| American Nurses Association | Not for Profit | Silver Spring |
| American Society of Health-System Pharmacists | Not for Profit | Bethesda |
| American Speech Language Hearing Association | Association | Gaithersburg |
| Aronson & Company | Business Services | Rockville |
| Athena Consulting | Consulting | Germantown |
| Avendra, LLC | Construction | Rockville |
| B.F Saul company Hotel Division | Real Estate & Construction | Bethesda |
| Balducci’s | Retail | Bethesda |
| Bioqual | Healthcare, Pharma & Biotech | Rockville |
| BioReliance | Healthcare, Pharma & Biotech | Rockville |
| Boland Trane Services, Inc. | Wholesale & Distribution | Gaithersburg |
| Broadsoft, Inc. | Technology | Gaithersburg |
| Century Distributors | Distribution | Rockville |
| Certified Building Services, Inc. | Other | Derwood |
| Choice Hotels International, Inc. | Recreation & Leisure | Rockville |
| Clark Construction Group | Construction | Bethesda |
| CNSI | Technology | Rockville |
| Coakley and Williams Construction | Real Estate & Construction | Bethesda |
| Community Pool Management | Real Estate & Construction | Rockville |
| Concrete General, Inc. | Real Estate & Construction | Gaithersburg |
| Coventry Health Care | Insurance | Bethesda |
| Criswell Chevrolet | Retail | Gaithersburg |
| Cystic Fibrosis Foundation | Non-Profit | Bethesda |
| Danya International | Healthcare | Silver Spring |
| Dataprise, Inc. | Technology | Rockville |
| Development Alternatives Inc. (DAI) | Business Services | Bethesda |
| Digital Management (DMI) | Technology | Bethesda |
| Discovery Communications | Entertainment | Silver Spring |
| Eagle Bancorp, Inc. | Banking | Bethesda |
| Emergent Biosolutions | Healthcare, Pharma & Biotech | Rockville |
| Envelopes Unlimited, Inc. | Manufacturing | Rockville |
| Europ Assistance USA | Travel Services | Bethesda |
| Federal Realty Investment Trust | Real Estate & Construction | Rockville |
| Financial Industry Regulatory Authority | Association | Rockville |
| First-Potomac Realty Trust | Real Estate & Construction | Bethesda |
| Forrester Construction Company | Construction | Rockville |
| Foulger-Pratt Management | Real Estate & Construction | Rockville |
| Francis O Day Co | Real Estate & Construction | Rockville |
| Gali Service Industries | Janitorial Services | Rockville |
| GEICO | Insurance | Chevy Chase |
| Global Exchange Services (GXS) | Technology | Gaithersburg |
| Grunley Construction Co., Inc. | Construction | Rockville |
| Henry M Jackson Foundation For the Advancement of Military Medicine | Healthcare | Rockville |
| Highway and Safety Services, Inc. | Real Estate & Construction | Gaithersburg |
| Holy Cross Hospital of Silver Spring | Healthcare | Silver Spring |
| Host Hotels & Resorts, Inc. | Recreation & Leisure | Bethesda |
| Howard Hughes Medical Institute (HHMI) | Non-Profit | Chevy Chase |
| Hughes Communications | Telecommunications | Germantown |
| IMSG | Technology | Rockville |
| Intelligent Automation | Technology | Rockville |
| Iq Solutions, Inc. | Technology | Rockville |
| JBG Companies | Real Estate & Construction | Chevy Chase |
| JBS International | Technology | N. Bethesda |
| J Craig Venter Institute | Education | Rockville |
| Kaiser Permanente of the Mid-Atlantic States | Healthcare, Pharma & Biotech | Rockville |
| Kay Apartment Communities | Real Estate & Construction | Silver Spring |
| Kids After Hours | Education | Silver Spring |
| Kirlin Group | Real Estate & Construction | Rockville |
| Landon School | Education | Bethesda |
| Lerch, Early & Brewer | Consulting | Bethesda |
| Lerner Companies | Real Estate & Construction | Rockville |
| Lockheed Martin Headquarters | Technology | Bethesda |
| Macrogenics | Healthcare, Pharma & Biotech | Rockville |
| Manders Decorating Company | Real Estate & Construction | Silver Spring |
| Mariam, Inc. (dba DARCARS) | Retail | Silver Spring |
| Marriott International | Hospitality | Bethesda |
| McFall and Berry Landscape Mgt | Landscaping | Brookeville |
| Meso Scale Diagnostics | Healthcare, Pharma & Biotech | Rockville |
| Miller and Long | Real Estate & Construction | Bethesda |
| Novavax | Healthcare, Pharma & Biotech | Rockville |
| OMV Medical | Healthcare, Pharma & Biotech | Takoma Park |
| Pleasants Construction (Core Commercial) | Real Estate & Construction | Clarksburg |
| Potomac Minute Maids | Janitorial Services | Rockville |
| Qiagen (North American HQ) | Biotechnology | Germantown |
| RainKing Solutions | Technology | Bethesda |
| Red Coats, Inc. | Janitorial Services | Bethesda |
| Retail Services and Systems (d/b/a Total Wine and More) | Business Services | Potomac |
| Ridgewell’s, Inc. | Other | Bethesda |
| RMA Worldwide Chauffeured Transportation | Transportation | Rockville |
| Roberts Oxygen | Manufacturing | Rockville |
| Ruppert Landscape | Landscaping | Laytonsville |
| Sandy Spring Bank | Banking | Olney |
| SEEC | Not for Profit | Rockville |
| Shapiro and Duncan | Real Estate & Construction | Rockville |
| Shulman Rogers | Law Firm | Potomac |
| Singleton Electric Co | Real Estate & Construction | Gaithersburg |
| Smokey Glen Farms | Agriculture | Gaithersburg |
| Social & Scientific Systems, Inc. | Technology | Silver Spring |
| Sodexo (North American HQ) | Food & Beverage | Gaithersburg |
| Strathmore Hall Foundation, Inc. | Recreation & Leisure | North Bethesda |
| Synergy Enterprises | Technology | Silver Spring |
| Technical Resources International, Inc. | Consulting | Bethesda |
| Terrapin Systems | Technology | Rockville |
| Thales Defense & Security, Inc. | Electronics | Clarksburg |
| The ARC of Montomery County | Other | Derwood |
| The Calvert Group | Finance | Bethesda |
| The Emmes Corporation | Technology | Rockville |
| The RitzCarlton Hotel Company | Recreation & Leisure | Chevy Chase |
| TV One | Entertainment | Silver Spring |
| Ultra Electronics 3eTI | Defense/Aviation | Rockville |
| United Therapeutics | Biotechnology | Silver Spring |
| US Pharmacopeia | Healthcare, Pharma & Biotech | Rockville |
| Wedding Wire | Technology | Chevy Chase |
| Westat | Consulting | Rockville |
| Zenimax Media | Consulting | Rockville |