The sitcom returns to Channel 4 for its seventh series on November 26 at 10pm, but is available for online viewing from today.
Writing on Twitter, Mitchell said: "Apparently the 1st episode of Peep Show series 7 is already available on 4OD... I don't like that very much.
"I'm tweeting about it so you know but it makes me sad for TV as a medium and so I advocate watching it next Friday on Channel 4 at 10pm. Either way, I hope you enjoy it."
He added: "To clarify, I don't mind it subsequently being available on 4OD - that's all good - but think that, as a TV show, it should premiere on TV.
"I'm not angry about it. It's just not how things would be organised if it were up to me."
When one follower suggested that the 4OD clicks would count for viewing figures, Mitchell replied: "I suppose I'm just old-fashioned... I just worry that, if TV dies, internet revenues will not sustain proper programme-making. It'll all be skateboarding kittens."
The actor, who appears in the sitcom alongside Robert Webb, also confirmed that shooting on the series was completed around six weeks ago.
> A quick peep at new Peep Show!