The actress, 31, who is the daughter of Phil Collins and Jill Tavelman, has reportedly been dating director McDowell, the son of Malcolm McDowell and Mary Steenburgen, for a year.
She shared a photo on Instagram of them kissing, in which she is showing off an engagement ring.
She wrote: “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, and the TV adaptation of Les Miserables, while McDowell has directed films including The One I Love and The Discovery.
The couple have 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…”
