J. Mark Barnett, MD

Medical Oncologist / Hematologist

J. Mark Barnett, MD | Oncologist at Rocky Mountain Cancer Centers


1760 E. Ken Pratt Blvd. Ste. 302
Longmont, CO 80504


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About J. Mark Barnett, MD

Dr. Mark Barnett is board certified in medical oncology and hematology. He completed his fellowship training at the Brown University program in Providence, Rhode Island. He has practiced in the Longmont community since 2006. He lives in Longmont with his wife and three children and is honored to continue to serve the Longmont community. Dr. Barnett has over 15 years of experience in medical oncology and hematology and has particular interest in the areas of gastrointestinal and genitourinary malignancies. He is an advocate for cancer research, and actively participates in recruiting patients for clinical trials. He has received several awards recognizing his commitment to patient care including the Longmont Physician Service Excellence award. He currently serves on the partnership board for Rocky Mountain Cancer Centers. He actively collaborates with colleagues in a multidisciplinary approach to maximize patient outcomes from cancer treatment. He is known throughout the community as a very approachable and compassionate physician.

Advanced Practice Providers Working With Dr. Barnett

Board Certifications

Medical Oncology


  • Blood Cancers
  • Gastrointestinal Cancers
    • Anal Cancer
    • Colorectal Cancer
    • Esophageal Cancer
    • Liver Cancer
    • Pancreatic Cancer
    • Stomach Cancer
  • Genitourinary Cancers
    • Adrenal Cancer
    • Bladder Cancer
    • Kidney Cancer
    • Penile Cancer
    • Prostate Cancer
    • Testicular Cancer



Medical Oncology & Hematology, Brown University School of Medicine, Providence, RI


Internal Medicine, Brown University School of Medicine, Providence, RI

Medical School

Southern Illinois University School of Medicine, Edwardsville, IL

Patient Care Philosophy

I believe in bringing my full attention and focus to each patient at every visit. I believe practicing medicine is an honor that gives us the opportunity to experience the wonderful humanity and resilience of our patients. I learn a great deal from my patients and getting to know each of them personally is the best part of my work.


Additional Information

American Society of Clinical Oncology
Colorado Medical Society
Longmont United Hospital Cancer Committee, Chairman