Galecki was born in Bree, Belgium, to American parents of Polish, Irish, and Italian descent. His mother, Mary Louise (Mary Lou) Noon, was a mortgage consultant, and his father, Richard Galecki, was a member of the U.S. Air Force stationed in Belgium, and also worked as a rehabilitation teacher. Galecki is the eldest of three children with a brother, Nick, whom he describes as a "mechanical genius" in the automotive industry, and a sister, Allison. Galecki grew up in Oak Park, Illinois.
During an interview with New Zealand radio station ZM, Galecki recalled his childhood relationship with his mother. As a child, he was well known for making up long stories and tales. In such situations, his mother used to make him play the "quiet game", where he had to see how long he could go without talking. He also recalled that despite being a loving mother, she was also very tough. One phrase she would lovingly use was, "I love you, now get out."
Galecki made his acting debut in the 1987 CBS miniseries Murder Ordained with JoBeth Williams and future Roseanne co-star John Goodman. In 1989, Galecki portrayed Rusty Griswold in National Lampoon's Christmas Vacation. In 1990, he was cast as Danny Nash, the son of Robert Urich's lead character, on the NBC comedy American Dreamer; the following season, he was a regular cast member on the ABC sitcom Billy, a spin-off from Head of the Class. He appeared in one episode of Blossom in 1991, opposite his future The Big Bang Theory co-star Mayim Bialik.
During his run on Billy, Galecki began making guest appearances on the hit ABC sitcom Roseanne as the younger brother of Mark Healy (Glenn Quinn), who began a relationship with Darlene Conner (Sara Gilbert). Introduced in his first appearance as Kevin Healy, his name was soon changed to David. After a few more guest shots, Galecki was named a permanent Roseanne cast member from the fall of 1992 onward, after the cancellation of Billy. He would remain in the role of David Healy until the end of Roseanne's run in 1997, with the character eventually marrying Darlene and fathering a child with her.
Galecki also appeared in the 1995 music video for the Dave Matthews Band song "Satellite". Galecki was cast in the 1997 summer slasher I Know What You Did Last Summer and the 1998 dark comedy The Opposite of Sex. In the year 2000, Galecki reunited with Opposite of Sex director Don Roos, who cast him in Bounce.
Galecki continued his film streak with roles in Suicide Kings (1997) and brief bit parts in high-profile films such as Bean (1997), and a minor supporting part in Vanilla Sky (2001). Galecki played a young delinquent in A Family Torn Apart, a 1993 TV movie based on a true story about a serial killer. He was also in the 2003 film Bookies, a comedy thriller film written by Michael Bacall and directed by Mark Illsley. The story revolves around the lives of four college students. Galecki appeared as a golfer in a 2005 episode of My Name Is Earl entitled "Stole Beer from a Golfer".
In 2005, Galecki played Mark Corrigan in a pilot for a US adaptation of the British sitcom Peep Show. He played a character named Trouty on TBS's sitcom My Boys, and the half-brother of the main characters, sisters Hope Shanowski and Faith Fairfield, in the sitcom Hope & Faith. In late 2006 and early 2007, Galecki appeared in the Broadway play The Little Dog Laughed as a prostitute called Alex. "At its core, the play is about what we all sacrifice to be successful, whatever our careers or goals," he explains. Galecki originated the role earlier in 2006 at Second Stage Theatre. The widely popular comedy by Douglas Carter Beane had a successful run at Second Stage and moved to the Cort Theatre. The play closed on February 18, 2007, with Galecki taking home a 2007 Theater World Award for his performance.
Galecki plays Leonard Hofstadter in the CBS sitcom The Big Bang Theory, which debuted September 24, 2007. He was originally asked to play the role of Sheldon Cooper, but he said he was better suited for the role of Leonard Hofstadter. During his role as David Healy, Sara Gilbert played the role of his longtime girlfriend and later wife, Darlene Conner. She has appeared as Leslie Winkle, a colleague in the sitcom. Laurie Metcalf, formerly cast in the role of Roseanne's sister Jackie Harris, has appeared as Mary Cooper, mother of Leonard's roommate, Sheldon. Galecki is a cellist, a talent which has been used on the show. As of August 2013, Galecki and his Big Bang costars Kaley Cuoco and Jim Parsons each earned $325,000 per episode. As of August 2014, the three costars will earn $1 million per episode.
Galecki appeared briefly in the 2008 superhero comedy Hancock, alongside Will Smith and Jason Bateman. In July 2011, he played a parody version of himself in three episodes of Entourage. He also appeared in the film In Time (2011) with Justin Timberlake and Amanda Seyfried.
On April 28, 2017, it was reported that a revival of Roseanne was in the works with most of the original cast, including Galecki, returning for the eight part series. On May 16, 2017, it was confirmed that the eight episodes would air mid-season in 2018 on ABC.
While working on The Big Bang Theory, Galecki dated co-star Kaley Cuoco for about two years until December 2009 while the two also played a couple on the show. Cuoco told CBS Watch that they have remained on good terms since ending their relationship. Galecki was in a relationship with actress Kelli Garner for some time before it ended in 2014.
Galecki owns 360 acres (150 ha) of land in Santa Margarita, California. His property included vineyards and a log cabin. In late June 2017, Galecki's ranch was destroyed in a major wildfire known as the Hill Fire. Galecki, in a media statement, said wildfire was "the threat ... we live with constantly, which may seem crazy to some but we do so because living in our beautiful, rural area makes it worthwhile."