McAdams was born in London, Ontario, and grew up in nearby St. Thomas. Her father, Lance, is a truck driver, and her mother, Sandy, is a nurse. She has a younger sister, Kayleen and younger brother, Daniel.

At the age of four, she took up competitive figure skating. She took up acting at age thirteen at a summer theater camp. When the company extended to a year-round company McAdams was invited to continue with them. During her senior year at the York University in Toronto, she played a child in The Piper - a workshop led by a creative team from Toronto's Necessary Angel Theatre Company. She attended Central Elgin Collegiate Institute in St. Thomas from grade nine to OAC, and later graduated from York University with honours and a B.F.A degree in theatre. She also attended Original Kids Theatre Company in London, Ontario, as well as David Rothenberg's on-camera acting class in Toronto with fellow actors Scott Speedman, Kenneth Mitchell, Polly Shannon and David Sutcliffe.

She had a large role in the film The Hot Chick, alongside Rob Schneider, but her career really took off when she starred as "Regina George", "queen-bee" and the resident bitch of her school, in the 2004 hit Mean Girls. She starred opposite Hollywood It-Girl Lindsay Lohan. McAdams was told to partly model the character of Regina George after Alec Baldwin's performance in Glengarry Glen Ross (1992). Mean Girls got McAdams's name into many homes in the United States, and she won rave reviews for her performance.

McAdams had previously appeared in the Canadian television series "Slings and Arrows", alongside Paul Gross. She played a main role in the first season, but due to her rising stardom was written out of the second season, appearing in only the first episode.

She followed Mean Girls with the film adaptation of Nicholas Sparks' novel The Notebook, alongside Ryan Gosling. The pair would later continue their on-screen romance, rumoured to have started after they shared a passionate kiss onstage when accepting the MTV movie award for "best kiss".

In 2005, she played Owen Wilson's love interest in Wedding Crashers, and starred in the suspense thriller Red Eye. Her most recent role is in the comedy The Family Stone where she stars alongside Diane Keaton, Craig T. Nelson, Sarah Jessica Parker, Dermot Mulroney, Luke Wilson and Claire Danes.

McAdams abruptly opted out of a cover shoot for Vanity Fair alongside two other Hollywood "It girls," Scarlett Johansson and Keira Knightley, upon finding out it was to be nude. Tom Ford, artistic director for the cover, instead appeared with them. "She did want to do it, and then when she was on the set I think she felt uncomfortable, and I didn't want to make anybody feel uncomfortable," Tom Ford said on ABC's Good Morning America.

McAdams lives in Toronto. She is a former vegetarian, and is currently dating fellow Canadian actor Ryan Gosling. Both were born in St. Joseph's Hospital in London, Ontario.

McAdams received popular acclaim for both roles and was honoured for her work, most recently a record number of five nominations, and three wins, at the MTV Movie Awards in 2005. McAdams beat competition including Beyonc� Knowles, Jessica Alba, and Jessica Simpson. She was named #14 on the Maxim magazine Hot 100 of 2005 list and #17 in the 2006 Hot 100 issue. In December 2005, she was #1 on MuchMusic's "Who To Do: 20 Sexiest Girls" list. McAdams was also voted the #101 on FHM 100 Sexiest Women in the World 2005.

McAdams was nominated for a Genie Award, the Canadian equivalent of the Academy Award, for her role in Perfect Pie. She was also nominated for the BAFTA's Orange Rising Star award which acknowledges the new talents in the acting industry. She was beaten by James McAvoy.

She hosted Academy of Motion Picture Arts and Sciences's 2006 Scientific and Technical Awards. McAdams received a Civic Award for her contribution to culture from the city of St. Thomas in 2006.