Culkin was born in New York City, the son of Patricia Brentrup and Christopher 'Kit' Culkin, a former stage actor with a long career on Broadway. Culkin is the youngest of seven children; he has four brothers: Shane Arliss Culkin (born 1976), Macaulay Culkin (born 1980), Kieran Kyle (born 1982), and Christian Patrick Culkin (born 1987), and two sisters: Dakota Ulissa Culkin (1979-2008) and Quinn Kay Culkin (born 1984). Culkin is the nephew of actress Bonnie Bedelia, who is his father's sister.

Culkin began acting by playing alongside his elder brothers, amongst them Macaulay and Kieran, often as the younger versions of their characters. He first appeared in a photograph as a baby in The Good Son (1993), then as Young Richie in Ri¢hie Ri¢h (1994) (Richie being played by his brother Macaulay) and in 2002 played 10-Year-Old Igby in Igby Goes Down (Igby was played by his older brother Kieran).

Culkin's breakthrough role was in You Can Count On Me, opposite Laura Linney, a role for which he received much praise and a Young Artist Award. Since then, Culkin has appeared in numerous films, Signs being the most famous, in which he starred alongside Mel Gibson and Joaquin Phoenix. As a teenager, Rory started moving into more independent films, such as The Chumscrubber and Down in the Valley. He took a leading role in Mean Creek, a 2004 independent film about a group of teenagers that plan to get revenge on a bully. The entire youth cast won an Independent Spirit Award for this film. Since his Young Artist Award win for You Can Count on Me, Culkin has received three more nominations. He also had a guest role in the Law & Order: Special Victims Unit episode "Manic", and in an episode of the Twilight Zone called "Azoth the Avenger Is a Friend of Mine" alongside Patrick Warburton. In May 2010, Culkin was cast for the upcoming slasher film Scream 4.