Ryan's Jeep careened off the Pacific Coast Highway and landed on the rocks below, according to TMZ.com. Ryan, 50, was pronounced dead at the scene.
Ryan performed cosmetic surgery on many stars but was best known for his 10-hour procedure on Montag, 23, last Nov. 20, during which she had some 10 alterations done.
Asked by PEOPLE at the time whether such extensive work was a good idea, Ryan replied: "My first concern is for the safety of the patient, so I would never push the envelope. Heidi's a young, healthy girl; she was cleared medically. It was well within the realm of safety."
For her part, Montag trusted Ryan fully. "Dr. Ryan is the same doctor who did my nose job and breast augmentation in 2007, so I knew I'd be in good hands," she told PEOPLE last year. "In the three years since he last operated on me, we've had at least 20 meetings so we could look at photos of me and of what I want to look like."