The midfielder, who joined the Reds when our women's team was formed in July 2018, has made 29 appearances in all competitions for the club so far, scoring 16 goals.
“Mollie is a great talent and had a fantastic season with us last year,” said manager Casey Stoney.
“The competition for places is at such a high level this season but it’s important to get minutes on the pitch so this loan move will be really beneficial for her development.
“Sheffield United have been in great form this season and are currently battling for a place in the Super League next year, so it’s a perfect opportunity for Mollie with her talent and experience.