The 27-year-old submitted a move-in application to the Blues last summer, however, however, she left her contract at the end of her contract in June.
He made appearances with Birmingham in two seasons.
“I wanted to come here to win trophies and make history,” Staniforth told United’s website.
“I am pleased to have signed with Manchester United; it’s the biggest club in the world and I’m very proud to be a part of it.”
Staniforth, who has been voted 16 times by England, joins a United team that finished fourth in the Women’s Super League in its first season on the most sensible flight.
Coach Casey Stoney said he was “delighted” with the arrival of former Sunderland midfielder.
“Lucy enjoys women’s football a lot, having made her debut in the absolute category at 16, and I know she can use that pleasure for us on and off the field,” she added.
After last season shortened by the coronavirus pandemic, the 2020-2021 crusade is expected to begin in September.