Ryan was 50 years old.
He opened his private practice in 1994 and took care of several celebrities including musician Vince Neil, actor Lorenzo Lamas and reality star Adrianne Curry. He not only had a celebrity clientele, but was considered a celebrity himself.
Montag took to herTwitter to share her reaction to his death.
"I am devastated," the 23-year-old wrote Tuesday. "He was the most amazing person I have ever known. He was an angel and changed my life and the lives of everyone he met. He was the most brilliant talented surgeon who will ever exist. Dr. Frank Ryan changed the world."
She added, "My thoughts and prayers go out to his mother, family, friends, and anyone who was ever blessed enough to meet him. He is in a better place."
Many said he wasn't only their doctor, but also their friend.
On Tuesday evening, friends, family and fans gathered at Malibu's Zuma Beach for a candlelight vigil.
Janice Dickinson, a former model and reality TV star, shared a story and praised him for his work.
"I used to say Frank Ryan was the Bentley of surgeons," she said.
Ryan will also be remembered for his philanthropic work.
"Once a week he had a clinic that would remove tattoos from former gang members," said Stanton Dodson, a friend of Ryan's. "He worked with Oscar De La Hoya with the fights, he would work on other boxers and would never take money for it if they ever got a cut or an injury."
An autopsy was performed Tuesday but the exact cause of death has not been released.
The cause of the crash is under investigation.
The Associated Press contributed to this report.