McAdams was born and raised in a small town outside of London, Ontario. She developed an interest in performing at a very young age, and at twelve, McAdams participated in Original Kids Theatre Company productions. She was directing children's theatre productions through her teenage years, and was involved in school stage productions, even winning a performance award at the Sears Ontario Drama Festival.
McAdams had intended to take Cultural studies at the University of Western Ontario before being persuaded by her drama teacher that a professional acting career was a viable option. She enrolled in York University's four-year theatre program and graduated with a Bachelor of Fine Arts honours degree in 2001. While at university, McAdams worked with the Toronto-based Necessary Angel Theatre Company.
In 2001, McAdams made her television debut in the MTV pilot Shotgun Love Dolls as Beth Swanson. She spent some time working in Canadian television and film productions, such as the drama film Perfect Pie (for which she received a Genie Award nomination), the comedy film My Name Is Tanino and the comedy series Slings and Arrows (for which she won a Gemini Award). In 2002, she made her Hollywood film debut in the comedy The Hot Chick. McAdams rose to fame in 2004 with her roles in the comedy Mean Girls and the romantic drama The Notebook. In 2005, she starred in the romantic comedy Wedding Crashers, the psychological thriller Red Eye, and the family comedy-drama The Family Stone. She was hailed by the media as Hollywood's new "it girl", and received a BAFTA nomination for Best Rising Star.
After a two-year career break, McAdams starred in two limited release films in 2008: the film noir Married Life and the road trip comedy-drama The Lucky Ones. She returned to prominence in 2009 by appearing in the political-thriller State of Play, the science-fiction romantic drama The Time Traveler's Wife and the mystery action-adventure film Sherlock Holmes. In 2010, McAdams appeared in her first star vehicle, the comedy Morning Glory.
In 2011, she starred in Woody Allen’s Midnight In Paris, which earned McAdams a SAG nomination for Outstanding Performance by a Cast in a Motion Picture. That same year, she reprised her role as Irene Adler in Sherlock Homes: A Game of Shadows opposite Robert Downey Jr.
2015 proved to be quite a busy year for McAdams: She starred in the second season of the HBO crime drama True Detective; played journalist Sacha Pfeiffer in the drama Spotlight, for which she was nominated for the Academy Award for Best Supporting Actress; starred in Cameron Crowe’s romantic comedy Aloha, alongside Bradley Cooper and Emma Stone; starred opposite Jake Gyllenhaal in the drama Southpaw; and voiced The Mother for the animated picture The Little Prince.
In 2016, McAdams starred opposite Benedict Cumberbatch in the Marvel film Doctor Strange.
McAdams is an environmentalist, and she ran an eco-friendly lifestyle website called GreenIsSexy.org with two of her friends, from 2007 to 2011.
She volunteered in Biloxi, Mississippi and Louisiana in fall of 2005, as part of the clean-up effort following Hurricane Katrina.
In 2006, McAdams took part in the "Day Without Immigrants" demonstration in Los Angeles, protesting the federal government's attempts to further criminalize illegal immigrants living in the United States.
She appealed for donations during the Canada for Haiti telethon in 2010.
She was involved in Matter of Trust's "hair boom" efforts following the 2010 Gulf of Mexico oil spill.
In 2011, McAdams supported Foodstock, a protest against a proposed limestone mega quarry in Melancthon, Ontario. That same year, she also attended the Occupy Toronto demonstration.
In 2013, she filmed two promotional videos for the Food & Water First Movement, aiming to preserve prime farmland and source water in Ontario, Canada. She also volunteered with Habitat for Humanity in her hometown of St. Thomas.[
In 2014, she narrated the feature documentary Take Me To The River, which investigates what is being done to try to save iconic rivers.
She has also worked with charities including the Sunshine Foundation of Canada, the Alzheimer's Association, the READ Campaign, and United Way of Canada.
When in Toronto, Rachel McAdams can often be seen bicycling around the city.