Green-Ellis was signed by the Patriots as an undrafted free agent on May 1, 2008. He was waived by the team on August 30, 2008, and signed to the team's practice squad on September 1, 2008. He was promoted to the active roster on October 11.
Green-Ellis made his NFL debut on October 12, 2008, recording one catch for October 20, 2008, the same day that the Patriots placed fellow running back Laurence Maroney on injured reserve, Green-Ellis had his first NFL start in the Patriots' 41-7 victory against the Denver Broncos on Monday Night Football. Green-Ellis had 65 yards on 13 rushes in the second half, and scored his first touchdown on a 1-yard run, entering the end zone untouched. During Sunday Night Football's November 2, 2008 broadcast, Al Michaels and John Madden noted that his Patriots teammates had nicknamed Green-Ellis "Law Firm," on account of his multi-part name.