Clooney, an Irish American, was born in Lexington, Kentucky. His mother, Nina Bruce (née Warren), is a former pageant queen, while his father, Nick, is a journalist, anchorman, game show and American Movie Classics host, and – in later years – an aspiring politician from the state of Kentucky. He also has an older sister, Adelia (aka Ada). His cousins include actors Miguel and Rafael Ferrer, who are the sons of his aunt, singer Rosemary Clooney, and actor José Ferrer. He is also related to another singer, Debby Boone, who married José and Rosemary's son, Gabriel Ferrer. From an early age, Clooney would hang around his father's sets, often participating in shows, where he proved to be a crowd favorite. Clooney began his education at the Blessed Sacrament School in Ft. Mitchell, Kentucky. Spending part of his childhood in Ohio, he attended St. Michael's School in Columbus, and the Western Row and St. Susanna schools, both in Mason. There he developed an interest in theater. Eventually, his parents moved to Augusta, Kentucky, where one of his best friends was Steve Colemire. He went to Augusta High School where he was the lead in several plays. Clooney has stated that he earned all A's and a B in school, and was an enthusiastic baseball and basketball player. He tried out with the Cincinnati Reds in 1977 to play professional baseball, but was not offered a contract. He did not pass the first round of player cuts. He attended Northern Kentucky University from 1979 to 1981 and, very briefly, the University of Cincinnati, but did not graduate from either.

His first major role came in 1984 in the television medical comedy-drama E/R. Though it took place in a hospital, it should not be confused with ER, which Clooney more famously starred in a decade later. Additionally, he played a handyman on the series The Facts of Life. He played Bobby the detective on one episode of The Golden Girls. His first significant break was a semi-regular supporting role in the sitcom Roseanne, playing Roseanne Barr's overbearing boss Booker Brooks, followed by the role of a construction worker on Baby Talk and then as a sexy detective on Sisters. Clooney achieved stardom when he was selected to play Dr. Doug Ross on the hit NBC drama ER from 1994 to 1999. Clooney was also partnered with Deborah Leoni in their production company Mirador Entertainment.

Prior to his success on ER, he met Grant Heslov, a later close friend with whom he co-wrote Good Night, and Good Luck. Heslov was also the president of television at Section Eight Productions, Clooney and director Steven Soderbergh's production company. In August 2006, Clooney and Heslov started a new company: Smokehouse Pictures. Clooney said in an interview that he was driving an RV through the country with Heslov, who, at the time, was getting over a broken engagement, when he got a phone call from his agent telling him that NBC just picked up ER for a full season. Clooney said, "I think I just got my career." It has been rumored that Clooney was the one to have circulated the videotape of Jesus vs. Santa (the video greeting card that gave birth to South Park) around the Los Angeles area in 1995. The show's creators, Matt Stone and Trey Parker, invited him to play a role in the show as the voice of Stan Marsh's gay dog Sparky in the episode "Big Gay Al's Big Gay Boat Ride", a role with no dialogue except normal dog noises. He later appeared in the film South Park: Bigger, Longer & Uncut. Despite their history, the show's creators, Parker and Stone, lampooned Clooney for his outspoken political views in their feature film Team America: World Police. However, Clooney later said that he would have been offended if he hadn't been made fun of in the film. He was also mentioned in the episode "Smug Alert!", which mocks his acceptance speech at the 78th Academy Awards.

Clooney continued to star in movies while appearing in ER, his first major Hollywood role being From Dusk Till Dawn, directed by Robert Rodriguez. He followed its success with One Fine Day with Michelle Pfeiffer and The Peacemaker with Nicole Kidman, the latter being the initial feature length release from Dreamworks SKG studio. Clooney was then cast as the new Batman in Batman & Robin. In 1998, he starred in Out of Sight, opposite Jennifer Lopez. This was the first of many collaborations with director Steven Soderbergh. He also starred in Three Kings during the last weeks of his contract with ER. In 1999, Clooney left the cast of ER to pursue his film career full-time. He mentioned a few times that he would like to do a few cameos; to date, he has only done one.

After leaving ER, Clooney starred in major Hollywood successes, such as The Perfect Storm and O Brother, Where Art Thou?. In 2001, he teamed up with Soderbergh again for Ocean's Eleven, a remake of the 1960s Rat Pack film of the same name. To this day, it remains Clooney's most commercially successful movie, earning approximately US$ 444,200,000 worldwide. The film spawned two sequels starring Clooney, Ocean's Twelve in 2004 and Ocean's Thirteen in 2007. In 2001, Clooney founded the production studio Section Eight with Steven Soderbergh. Clooney is generally considered Chief Actor. He made his directorial debut in the 2002 film Confessions of a Dangerous Mind, an adaptation of the autobiography of TV producer Chuck Barris. Though the movie didn't do well at the box office, Clooney's direction was praised among critics and audiences alike. In 2005, Clooney starred in Syriana, which was based loosely on former Central Intelligence Agency agent Robert Baer and his memoirs of being an agent in the Middle East. The same year he directed, produced, and starred in Good Night, and Good Luck, a film about 1950s television journalist Edward R. Murrow's famous war of words with Senator Joseph McCarthy. Both films received critical acclaim and decent box-office returns despite being in limited release. At the 2006 Academy Awards, Clooney was nominated for Best Director and Best Original Screenplay for Good Night, and Good Luck, as well as Best Supporting Actor for Syriana. He became the first person in Oscar history to be nominated for directing one movie and acting in another in the same year. He would go on to win for his role in Syriana. More recently, he appeared in The Good German, a film-noir directed by Soderbergh. The film is set in post-World War II Germany.

Clooney is one of only two people to have been given the title of "Sexiest Man Alive" twice by People Magazine, first in 1997 and again in 2006. Clooney also received the American Cinematheque Award in October 2006, an award that honors an artist in the entertainment industry who has made "a significant contribution to the art of motion pictures". On January 22, 2008, Clooney was nominated for Best Actor for his role in Michael Clayton, but lost to Daniel Day-Lewis for There Will Be Blood. Clooney is self-deprecating in interviews, telling STV in April 2008 that Leatherheads, one of his lightest movies, is a "cry for peace." In the same interview, when asked about reconciling George Clooney the actor and George Clooney the director, he said "there's a lot of ego there... so I just take it out on the actors." Clooney is represented by Bryan Lourd, Co-Chairman of Creative Artists Agency (CAA).

On July 8, 2005, news reports said that Clooney would be working with Cindy Crawford's husband Rande Gerber to design and build a new casino hotel in Las Vegas. On August 29, the same year, Clooney officially announced his involvement with the Las Ramblas Resort project. However, the project never came to fruition, and the property on which the resort was to be built was sold in June 2006. After serving as pitchman outside the U.S. for products like Fiat and Martini vermouth, Clooney lent his voice to a series of Budweiser ads beginning in 2005 (which were still running as of September 2007).[citation needed] In September 2007, Clooney defended his work when asked by an Italian journalist how he reconciled working in a Nestle advertisement for Nespresso with his criticism of multinational companies. After the success of Good Night, and Good Luck, Clooney said, he plans to devote more of his energy to directing. He has said that the directing industry is "a great industry to grow old in."[cite this quote] Clooney directed the film Leatherheads, in which he also stars. Clooney is creating and producing a television series for Showtime titled The Fall of Bob. The Fall of Bob is an half-hour, single-camera black comedy-drama about a man who is committing suicide while a lengthy flashback occurs of what happened before his death.