Born Tami Mikkelson in Iowa, Hoag grew up in a very small Minnesota town. Because her siblings were more than ten years older than her and there were not a lot of other children nearby, Hoag developed an active imagination, making up stories to entertain herself. In 1977 she married her high school sweetheart, Daniel Hoag, shortly before he finished college. However, she never had the opportunity to go to college herself, as they moved to a town without easy access to higher education. The couple were later divorced.
Before publishing her first novel, Hoag held varying jobs, including a stint as a photographer's assistant, training show horses, working at the circulation desk at a newspaper, and even selling designer bathroom accessories. She began her career as an author in 1988, writing category romances for the Bantam Books Loveswept Line. After several years of success in that field, Hoag switched her focus to single-title suspense novels. She has had thirteen consecutive New York Times bestsellers, including five in a 20-month span.  Her novel Night Sins became a TV miniseries starring Valerie Bertinelli and Harry Hamlin. Hoag has been invited to do a reading at one of Barbara Bush's literacy functions, and then had lunch with former President George H.W. Bush and Mrs. Bush at their home.
Hoag and three other authors who made the leap from romance to thrillers at roughly the same time (Eileen Dreyer, Elizabeth Grayson and Kimberly Cates) have formed a group they call the Divas. The group provides support and encouragement for each other, and Hoag often thanks them in the acknowledgement section of her books. 
Hoag owns horses, and often goes for a ride to combat writer's block. She has competed in dressage at a national level, but stopped competing after breaking five vertebrae in her back during a fall while trying out a horse for a friend. Hoag is fully recovered from her accident, and has returned to the competition arena. She currently lives in Malibu, California, and Wellington, Florida.