Kruszelnicki's background was hidden from him for a long time, with his mother having told him that she was Swedish and a Lutheran but she was, in fact, Polish and Jewish.

[4] His father Ludwik, a Polish Gentile, was turned in to the Gestapo for smuggling Jews out of Poland and was imprisoned at Sachsenhausen, a concentration camp used mainly for political prisoners.

As the end of World War II approached, Ludwik avoided execution by swapping identities with a dead person.

Kruszelnicki said his childhood as a refugee in Wollongong was difficult, and he was bullied at school; he said "anybody who was not an Irish Catholic was considered an outsider".

[9] After high school, he attended the University of Wollongong, completing a Bachelor of Science degree majoring in physics in 1968.

[11] He also worked as a filmmaker, car mechanic, TV weatherman and as roadie for Slim Dusty, Bo Diddley and Chuck Berry.

He designed a machine to test the strength of steel made for use in Melbourne's West Gate Bridge, which was under construction at the time.

His Master of Biomedical Engineering degree allowed him to design and build a machine to pick up electrical signals off the human retina to diagnose certain eye diseases.

[14] He talks fondly of his time as a children's doctor, though he left this profession after witnessing the first child die from whooping cough in twenty years.

Kruszelnicki presented a program on ABC TV in January 2025 titled Dr Karl's How Things Work.

[17] Kruszelnicki also often helps with other science and education Triple J promotions such as the Sleek Geek Week roadshow with Adam Spencer and Caroline Pegram.

[24] In 1981, he appeared on an Australian radio documentary about death and near-death experiences that aired on the ABC, And When I Die, Will I Be Dead?

[30] He puts the cause of his condition down to having an unhappy, lonely childhood, saying that it impeded the development of the part of his brain responsible for remembering faces.

[31] In the 2001 honours list, he was awarded the Centenary Medal "for major service in raising public awareness of the importance of science and technology".

[32] One of Kruszelnicki's more notable undertakings was his part in a research project on belly button fluff, for which he received the tongue-in-cheek Ig Nobel Prize in 2002.