Galecki also appeared in the films Prancer (1989), National Lampoon's Christmas Vacation (1989), Suicide Kings (1997), I Know What You Did Last Summer (1997), Bookies (2003), In Time (2011), and Rings (2017).

Galecki was one of the highest paid television actors in the world, with his role in The Big Bang Theory earning him approximately US$900,000 per episode between 2017 and 2019.

In 2018, he was estimated to be the world's second highest-paid male TV actor by Forbes (behind only his The Big Bang Theory co-star Jim Parsons), earning $25 million.

[4] Galecki is the eldest of three children with a sister, Allison, and a brother, Nick, who he describes as a "mechanical genius" in the automotive industry.

"[9] After working in the Chicago theatre scene as a young child, Galecki made his onscreen acting debut in the 1987 CBS miniseries Murder Ordained with JoBeth Williams and future Roseanne co-star John Goodman.

He appeared in one episode of Blossom in 1991, opposite his future The Big Bang Theory co-star Mayim Bialik.

He played a young delinquent in A Family Torn Apart, a 1993 TV movie based on a true story about a serial killer.

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 two children with her.

Galecki originated the role of Alex, a male prostitute, in the play The Little Dog Laughed by Douglas Carter Beane, in 2006 at Second Stage Theater.

Galecki played Leonard Hofstadter in the CBS sitcom The Big Bang Theory, which ran from 2007 to 2019 and was, for most of its run, among the top three most popular television comedies in the US.

[18] 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.