Her mother, Sandra (ne Gale), is a nurse, and her father, Lance, is a retired truck driver and furniture mover.McAdams has two younger siblings: Kayleen, a celebrity make-up artist, and Daniel.She has "a lot of Welsh" ancestry through a grandmother.

McAdams began figure skating at the age of four but turned down an opportunity to move to Toronto at the age of nine for pairs training.Skating then became merely "a hobby".She competed in the sport until the age of 18, winning regional awards.She has since said that skating prepared her for physical acting, because it trained her to be "in tune" with her body.

McAdams was educated at Myrtle Street Public School and, later, Central Elgin Collegiate Institute.She did not enjoy school and, to avoid attending, she often pretended to be sick.Nonetheless, she played an active role in student life.

In addition to playing sports, McAdams served on the student council, participated in the Crimestoppers program and was a member of the Peer Helping Team.She worked at a McDonalds restaurant during the summer holidays for three years.

McAdams first developed an interest in performing when she was seven.She attended both Disney and Shakespeare summer camps as a child.From the age of twelve, McAdams appeared in Original Kids Theatre Company, London productions. In her late teens, she directed children's theatre productions.She was also involved in school stage productions, most notably winning a performance award at the Sears Ontario Drama Festival.

After graduating from a four-year theatre program at York University in 2001, McAdams initially worked in Canadian television and film productions such as My Name is Tanino, Perfect Pie (for which she received a Genie Award nomination) and Slings and Arrows (for which she won a Gemini Award).

McAdams made her first onscreen appearance in Shotgun Love Dolls, a 2001 MTV pilot filmed during a spring break from York University.McAdams later earned a Genie Award nomination in her native Canada for her role as a teenage Wendy Crewson in Perfect Pie.Her first Hollywood movie was 2002's The Hot Chick which McAdams has described as a "huge milestone" in her career.McAdams found fame in 2004 with starring roles in the teen comedy Mean Girls and the romantic drama The Notebook.

In 2005, she appeared in the romantic comedy Wedding Crashers, the psychological thriller Red Eye and the family drama The Family Stone. She was hailed by the media as "the new Hollywood "it girl"and received a BAFTA nomination for Best Rising Star.

However, McAdams withdrew from public life in 2006 and 2007. During this time, she turned down leading roles in high-profile films such as The Devil Wears Prada. She made a low-key return to work in 2008, starring in two limited release films: the film noir Married Life and the road trip movie The Lucky Ones.

She returned to prominence in 2009 with appearances in the political thriller State of Play, the science-fiction romance The Time Traveler's Wife and the action-adventure film Sherlock Holmes. McAdams's first star vehicle was the 2010 comedy Morning Glory.

In 2006 McAdams received a Rising Star Award nomination from the British Academy of Film and Television Arts and hosted the Academy Awards for Technical Achievement.

McAdams' first film of 2011, Woody Allen's Midnight in Paris, opened the 2011 Cannes Film Festival and saw her reunite with Wedding Crashers's Owen Wilson. McAdams's second screen appearance of 2011 was a cameo role in the action-adventure sequel Sherlock Holmes: A Game of Shadows.

Two films starring McAdams were in competition for the Golden Lion at the 2012 Venice International Film Festival.To the Wonder, a romantic drama written and directed by Terrence Malick, and was Brian De Palma's Passion.

McAdams has completed filming About Time, a romantic comedy written and directed by Richard Curtis, on location in Cornwall and London. The film is scheduled for release on May 10, 2013.

McAdams is currently in Hamburg, Germany to film an adaptation of John Le Carr's espionage thriller A Most Wanted Man, directed by Anton Corbijn and co-starring Philip Seymour Hoffman, Robin Wright and Willem Dafoe.

McAdams is an environmentalist. She ran an eco-friendly lifestyle website GreenIsSexy.org with two friends for five years, from 2007 to 2011.Her house is powered by Bullfrog renewable energy. She travels around Toronto by bicycle but drives when in Los Angeles because it is "a harder town to cycle in."

McAdams sat on a TreeHugger/Live Earth judging panel in 2007.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.She is also a supporter of BluePlanetNetwork.org's clean drinking water initiative.

McAdams is also involved with other charitable and social causes. She volunteered in Biloxi, Mississippi in 2005, as part of the clean-up effort following Hurricane Katrina.

In 2006, she took part in the "Day Without Immigrants" demonstration in Los Angeles, protesting against the federal government’s attempts to further criminalize undocumented aliens living in the United States.

She appealed for donations during the Canada for Haiti telethon in 2010.In 2011, McAdams attended the Occupy Toronto demonstration.She has worked with charities including the Sunshine Foundation of Canada,the Alzheimer's Association, the READ Campaign,and United Way of Canada.

McAdams lives in the Harbord Village neighborhood of Toronto, Ontario, Canada. McAdams also holds a US green card.She practices kundalini yoga daily.She likes to garden and cook in her spare time and has said that she would open a restaurant if she were not an actress.

McAdams had a two-year relationship with Canadian actor Ryan Gosling from mid-2005 to mid-2007. They also briefly reunited in the summer of 2008.Following their split, Gosling described her as "one of the great loves of my life".She dated American actor Josh Lucas for a number of months in 2009.

McAdams is currently in a relationship with Welsh actor Michael Sheen, whom she met on the set of Midnight in Paris in July 2010.