The actress, 31, took to Instagram to share the happy news and posted shots of the proposal on Friday.
Writing on Instagram she said: “I’ve been waiting my lifetime for you and I can’t wait to spend our lifetime together…”
Collins is best known for her work in films such as Mirror Mirror and Love, Rosie, while McDowell has directed films including The One I Love and The Discovery.
The couple have recently been documenting their road trips on Instagram.
Celebrating McDowell’s birthday in July during a camping trip in northern California, Collins wrote on Instagram: “Today it’s your birthday and yet I feel like I’m the lucky one. You’re a true gift of a human @charliemcdowell. An incredible boyfriend and the best dog dad to @redforddog.
“You’ve opened my eyes and deepened my heart. You constantly inspire me to dream big and go outside of my comfort zone.
“Every day is an adventure with you and I can’t wait to explore more of the world together (when we can). I love you so much…”