"As" used as a conjunction

"As is mention above"

"As it is mentioned above"

Are both right? Which is more frequently used?

it is usually said ," as is mentioned above".

I think you made a typo, in any type of that phrase the word is always "mentioned" with the -ed at the end.  The reason it is always "mentioned" is because it is always in past tense when used in that phrase – you are calling attention to something already known, not introducing new information. 

"As it is mentioned above…" sounds archaic to me.  Better to drop the "it."