The 31-year-old, who has been dating McDowell for a year, took to Instagram to show off her sparkling engagement ring.
Revealing the news with a picture of the pair kissing, Collins captioned the post: “I’ve been waiting my lifetime for you and I can’t wait to spend our lifetime together.”
A number of the couple’s celebrity friends were quick to congratulate them on their happy news.
High School Musical star Ashley Tisdale wrote: “Congrats!” followed by a series of loveheart emojis.
Elsewhere, singer Ciara wrote: “Soooo happy for you! Love you Li Li boo! What a special day!”
Spinning Out star Kaya Scodelario wrote: “Yeayyy I’ve been waiting to see this post! Congratulations!”
While Collins has starred in a number of high profile roles, including Mirror Mirror, Extremely Wicked, Shockingly Evil and Vile and Tolkein, McDowell is an established director.
The Los Angeles born resident has directed episodes of Silicon Valley, Dear White People and Tales From The Loop. He is best-known for his debut movie The One I Love, which stars Elizabeth Moss.
The 37-year-old posted his own snap of Collins to his Instagram page, captioning the post: “In a time of uncertainty and darkness you have illuminated my life.
“I will forever cherish my adventure with you.”
It looks set to be a busy month for Collins.
The star, daughter of English musician Phil Collins, is about to appear as the lead in upcoming Netflix comedy-drama Emily in Paris.
The series, created by Sex and the City producer Darren Star, follows Emily (played by Collins) an American woman from the Midwest, who moves to Paris for a job opportunity and is left awed by the differences in French culture.