The outspoken Democrat, also said he was "saddened" by the "real attack" on US institutions that "hold truth to power".
Clooney called for Biden to quit the race in a New York Times opinion piece and argued the party should pick a new nominee at its August 2024 convention, saying the process would be "messy" but "wake up" voters in the party's favour.
Asked if he would write this again, he told US show CBS News Sunday Morning: "Yes, we had a chance. I wanted there to be, as I wrote in the op-ed, a primary.
"Let's battle test this quickly and get it up and going. I think the mistake with it being Kamala is that she had to run against her own record, and it's very hard to do if the point of running is to say, 'I'm not that person'. It's hard to do," he said.
"And so she was given a very tough task. I think it was a mistake, quite honestly.
"But we are where we are. We were going to lose more house seats, they say.
"To not do it would be to say, 'I'm not going to tell the truth'."
Biden ended his bid for re-election in July 2024 following mounting pressure to step aside after a faltering debate with Republican rival Donald Trump a month earlier.
His son, Hunter Biden, accused Clooney of exaggerating the former president's frailty.
"I could spend a lot of time debunking many of the things he said, because many things he said were just outright lies," Clooney said.
Speaking about the importance of a free press, Clooney said: "I want our government to reflect us at our best and to step in when needed and I am watching a real attack on all the basic institutions, and I'm saddened by that.
"And I've seen this pendulum swing so many times. People will talk about this being the worst time in the country for a lot of people, and it's bad," he said.
However, he pointed out "1968 was pretty rough, too".
"Every city in the United States was on fire. They killed Martin Luther King and Bobby Kennedy. The Tet Offensive. Vietnam War still raging. The capital was surrounded by armed guards to protect them," he said.
"We've been in tricky situations. We've had a civil war. We'll get through it, but I am saddened by it, and I think that we must protect the institutions that hold truth to power."
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