The 'Ocean's Eleven' actor was due to star in the new film, which tells the story of a CIA rescue operation to save six American diplomats held hostage in Iran in the 1970s, but he was forced to drop out because it clashed with his 2011 political thriller with Ryan Gosling.
However, Clooney admits he was happy to turn over the script to Affleck, who agreed to star in and direct 'Argo', reported New York Post.
"I was doing Ides of March and gave this (Argo) to Ben because we were ready to shoot... We felt really lucky he wanted to do it, and he did better than anyone could imagine," said Clooney.