Born in Belgium (where his father was stationed while serving in the US Air Force) and raised in Chicago, Galecki knew at an early age that he wanted to be an actor. By age seven, he had begun to land roles in local theater productions, appearing in shows like "Fiddler on the Roof", "Pippin" and "The Member of the Wedding". Galecki segued to the big screen as River Phoenix's younger brother in "A Night in the Life of Jimmy Reardon" (1988) and inherited the role of Rusty Griswold in 1989's "National Lampoon's Christmas Vacation". After twice playing the son of Robert Urich (in the 1990 NBC miniseries "Blind Faith" and on the short-lived NBC sitcom "American Dreamer"), he garnered sitcom success on "Roseanne". Galecki had previously worked with the comedienne playing her son in the ABC TV-movie "Backfield in Motion" (1991). He branched out to small screen dramas earning kudos for his turn as an adopted son accused of complicity in his parents' murder in the taut "A Family Torn Apart" (NBC, 1993) and as a manic-depressive youth in "Without Consent" (ABC, 1994).

Once the sitcom left the airwaves, Galecki returned to features in supporting roles as a motorcyclist in the comedy "Bean" as a murder witness in the hit thriller "I Know What You Did Last Summer" (both 1997). Critics praised his portrayal of a whiny, unwitting accomplice in the kidnapping of a mobster in "Suicide Kings" and the actor veered away from his cute image to play a mincing homosexual who stirs up trouble in "The Opposite of Sex" (both 1998). Galecki further branched out by selling the screenplay for "Snollygoster" (co-written with Chris Kletzien), about a group of college friends hiding a secret.