Garrett was under contract with the Cowboys until Jan. 14. Now a free agent, the Giants brought Garrett into their building the next day.
Garrett spent 10 years as head coach of the Cowboys. He's since been replaced by Mike McCarthy.
The Giants fired head coach Pat Shurmur on Dec. 30 and hired Joe Judge as his replacement. Shurmur was announced as the offensive coordinator for the Denver Broncos on Tuesday.