Mitchinson, from The Close in Brentwood, was out riding with a male friend in Ashwells Rd in Pilgrims Hatch when the accident occurred about 9.50am on Sunday.
The accident involved a white Toyota van driven by a 51-year-old man.
Mitchinson's friend was treated by ambulance crews at the scene before being taken to Queens Hospital, Romford, and later released.
One horse was killed in the crash and the second had to be destroyed, police said.
The driver is accused of causing death by careless driving. He has been released on police bail until March 19, while inquiries continue.
Road closures were put in place as officers investigated the crash.