The actor appeared as the pediatrician in the medical drama’s first five seasons from 1995 to 1999.
While ultimately coming good in the end, his character was depicted as a womaniser whose behaviour often took on a toxic light, especially when viewed decades later.
Despite this, Ross remains one of the show's best loved characters alongside Mark Greene (Anthony Edwards), John Carter (Noah Wyle) and Julianna Margulies (Carol Hathaway).
Clooney thinks he knows why this is, and in a new interview with Deadline, reflected upon his time on the show.
"I was a womaniser on the show and I was a drunk," he said, adding: “But, as long as at the end of every episode I’d be like ‘Don’t touch that kid,’ then I could get away with anything."
He said this knowledge of his time on ER helped when directing and starring in his new film, the Netflix drama The Midnight Sky.
In the film, Clooney plays a scientist named Augustine who is all alone in the Arctic on a depopulated Earth. However, he soon finds that he has a young girl for company named Iris (the eight year old newcomer Caoilinn Springall).
"I knew it gave me an opening as an actor to not have to try and make this guy necessarily likeable," he said. "That all I had to do was try to protect the girl and then everything else you could be as unhappy or as angry. He doesn’t smile at all or anything."
The Midnight Sky will be released on Netflix on 3 December. ER is available to binge on All 4.