Rachel Emily Nichols was born in Augusta, Maine, the daughter of Alison and James Nichols, a schoolteacher. She has English, French Canadian, German, Irish, Italian and Scottish ancestry. She attended and graduated from Cony High School in Augusta, where she competed in the high jump. She attended Columbia University in New York City, where she eventually graduated with a double major in mathematics and economics. She began modeling, launching a successful career with work for such high-profile companies as Guess?, Abercrombie & Fitch and L'Oréal.

Rachel moved into acting, snagging a role on the HBO situation comedy Sex and the City (1998) with her very first audition. She made her film debut as Jessica Matthews in the prequel Dumb and Dumberer: When Harry Met Lloyd (2003). Success continued with roles in such projects as The Amityville Horror (2005), The Inside (2005), Alias (2001), G.I. Joe: The Rise of Cobra (2009), Criminal Minds (2005) (a recurring role as FBI Agent Ashley Seaver) and Conan the Barbarian (2011). She played the lead role of police officer Kiera Cameron on the science fiction series Continuum (2012).