Williams won the first five games before the Italian finally got on the board, and that was only a minor delay before the fourth seed clinched the opening set after half an hour with a backhand winner.
It was her 18th winner in seven games compared to three from her opponent, and the pattern continued at the start of the second set.
Williams was the hot favourite to win the title before the tournament after her Wimbledon and Olympic success, and that has certainly not changed. She has dropped only 19 games in six matches, and blasted 38 winners compared to only six from Errani.
Williams said: "It's really awesome. This is what I wanted and what I dreamed of all year. I'm really happy, I'm in the final again, and I'm excited."
Errani still has a chance of winning US Open silverware when she and fellow Italian Roberta Vinci take on Czech pair Lucie Hradecka and Andrea Hlavackova in the women's doubles final.