Long, 18 has made four appearances for the Blades this season - mostly in the Johnstone's Paint Trophy and has been a regular backup on the substitutes bench during Danny Wilson's reign.
"We're delighted that George Long has committed his future to the club for the next few years, which means we keep a fantastic prospect, who has a great future ahead of him," Wilson told sufc.co.uk."It's now down to him to keep working hard on the training pitch and learn from Steve Simonsen and Paul Crichton."