Mendes told Total Film that he was initially sceptical of the casting for the series reboot Casino Royale, but has since been proven wrong.
"I was one of the people who said I didn't think he was the right casting," he said. "At the time I was asked in an interview and I said I'm not sure, I would have advised him not to do it. I was completely wrong.
"I bumped into him after Casino Royale and I was so excited [to see him as Bond]. To watch him go through that intense pressure of being cast as Bond with flying colours... it was great to see him come through that and prove the doubters wrong."
The 23rd Bond film also stars Naomie Harris, Ben Whishaw, Javier Bardem and Ralph Fiennes. A trailer for the movie is expected to premiere next month.
Skyfall is due for release on October 26 in the UK and November 9 in the US.
> James Bond 'Skyfall' Shanghai set video - watch
Watch Sam Mendes's Skyfall video blog below: