Boyle received an eight-month prison term, suspended for two years, and was ordered to complete 300 hours of community service.
Mitchinson and another rider were riding horses along Ashwells Road in Pilgrims Hatch on Sunday, January 16, last year when Boyle's Toyota van crashed into the animals.
Mitchinson was thrown from her horse and died instantly at the scene.
The other rider, a man, was also thrown from his horse but escaped serious injury. Both horses died as a result of the impact.
Inquiries by the Essex Police Serious Collision Investigation unit showed Boyle did not adjust his driving sufficiently to compensate for the bright sunlight at the time, resulting in him not seeing the horses on the road.
The senior investigating officer, Inspector Keith Whiting, said: "This case shows drivers must always have the highest level of concentration. The consequences of not taking care are massive.
"Our thoughts are with Mrs Mitchinson's family who live with her loss and I'm sure Mr Boyle lives with the thought of taking a person's life.
"I hope others will also learn and prevent such unnecessary deaths occurring in the future."