Greger went to college at Cornell University School of Agriculture, where as a junior he wrote informally about the dangers of bovine spongiform encephalopathy on a website he published in 1994. In the same year he was hired to work on mad cow issues for Farm Sanctuary, near Cornell, and became a vegan after touring a stockyard as part of his work with Farm Sanctuary. In 1998, he appeared as an expert witness testifying about Bovine spongiform encephalopathy when cattle producers unsuccessfully sued Oprah Winfrey for libel over statements she made about the safety of meat in 1996.
He enrolled in Tufts University School of Medicine, originally for its MD/PhD program, but he withdrew from the dual degree program and only pursued the medical degree. He graduated in 1999 as a general practitioner specializing in clinical nutrition. In 2001, he joined Organic Consumers Association to work on mad cow issues and spoke widely on the issue as cases of mad cow appeared in the US and Canada, calling mad cow "The Plague of the 21st Century."
In 2004, he launched a website and published a book critical of the Atkins Diet and other low carb diets.
In 2004, the American College Of Lifestyle Medicine was formed in Loma Linda, and Greger was a founding member as one of the first hundred people to join the organization.
In 2005, he joined the farm animal welfare division of the Humane Society as director of public health and animal agriculture. In 2008 he testified before Congress after the Humane Society released its undercover video of the Westland Meat Packing Company that showed downer animals entering the meat supply, which led to the USDA forcing the recall of 143 million pounds of beef, some of which had been routed into the nation's school lunch program.
In 2011, he founded the website NutritionFacts.org with funding from the Jesse & Julie Rasch Foundation.
In his lectures, videos, and writings about nutrition he tries to persuade people to change their eating habits from a Western pattern diet to a whole foods, plant-based, healthy diet—optimally to vegan diet—and says that such a diet can prevent and reverse many chronic diseases. He is critical of other doctors for not challenging their patients to adopt plant-based diets and to avoid animal-based products and criticizes the US government for giving watered-down advice about healthy eating in its guidelines, in order to protect the economic interests of food producers—especially those who make junk food or produce animal-based food.
Retired physician Harriet A. Hall, who is known for applying critical thinking to health claims, has written that, while it is well accepted that it is more healthy to eat a plant-based diet than a western pattern diet, Greger often overstates the known benefits of such a diet as well as the harm caused by eating animal products (for example, in a talk he claimed that a single meal rich in animal products can "cripple" one's arteries), and he sometimes does not discuss evidence that contradicts his strong claims.Heart Failure: Diary of a Third-Year Medical Student (2000)
Carbophobia: The Scary Truth Behind America's Low Carb Craze (2005).
Bird Flu: A Virus of Our Own Hatching (2007)
How Not To Die: Discover the Foods Scientifically Proven to Prevent and Reverse Disease (2015) (with Gene Stone)
Bird Flu: A Virus of Our Own Hatching received a favorable review which said it was "interesting and informative to both scientists and lay persons", but public health expert David Sencer was critical of the book, writing that it "focuses heavily on doomsday scenarios and offers little in terms of practical advice to the public" and that "a professional audience would quickly put [the book] aside for more factually correct sources of information".
How Not to Die made the New York Times Advice, How-to, and Miscellaneous best seller list for December 27, 2015, appearing at #6; for January 3, 2016, appearing at #11; and for January 10, 2016, appearing at #15.