She received instruction in fieldwork from chimpanzee specialist Jane Goodall, and began watching and recording the behavior of mountain gorillas. Her work took her to Zaire and then to Rwanda, where she opened the Karisoke Research Center. After years of patient observation, the gorillas came to know and trust her, and she found that she could sit in the midst of a group and even play with their young. She came to know the animals as individuals, and even gave them names.

In 1980, she went to England to attend Cambridge University, where she obtained a PhD in zoology. Afterwards she took a teaching position at Cornell University in New York where she wrote an account of her experiences in Rwanda. In 1983, it was published as Gorillas in the Mist. The next year she returned to the Karisoke Research Center to continue her fieldwork.

When her favorite gorilla, Digit, was killed by poachers for his hands, Fossey began a campaign against poaching. Her outspokenness, unfortunately, made her a target for violence. In 1985, she was found murdered in her cabin. No one has ever found her killer, although it is suspected that the person was a gorilla poacher.

Her legacy lives on in various organizations and societies dedicated to saving from extinction one of our closest primate relatives. Thanks to her work, the world's awareness of the endangered status of the mountain gorilla has increased, and the animals are now protected by the government of Rwanda and various international conservation organizations, including the Dian Fossey Gorilla Fund.

The Canadian orangutan expert, Dr. Birute Galdikas, has written of her colleague, Dian Fossey: "Perhaps in death Dian Fossey will achieve what eluded her in life: absolute protection for the mountain gorillas.  In her devotion to these animals, Dian will be remembered for generations as one of the heroes of the twentieth century. Without her work, it is doubtful that mountain gorillas would have survived to the twenty-first."