James, who was not in the starting lineup for Wednesday's 2-2 Champions League draw with Real Madrid, made her presence known early at Stamford Bridge with a goal in the 11th minute.
Her second was a rocket from the left side of the 18-yard box into the far corner, and she completed the hat-trick with a close-range finish in the 64th minute.
Aggie Beever-Jones and Sjoeke Nusken also scored for Chelsea, who lead Arsenal by six points but have played one game more.
Liverpool are fifth on 11 points after seven games.
The game at Stamford Bridge was Chelsea's first at home since the announcement Emma Hayes was taking over as manager of the United States women's team. She will leave four-times reigning WSL champions Chelsea at the end of this season.
