Garrett, 44, posted a 5-3 record since stepping in on November 8 for Wade Phillips, who was fired after the Cowboys had a 1-7 start to the campaign despite being a pre-season favorite to reach the playoffs.
"He passed the test," team owner Jerry Jones told a news conference at Cowboys Stadium.
The Cowboys, who began the season with hopes of becoming the first team to play in the Super Bowl at their home stadium, finished with their worst record since 2004.
Dallas's three losses since Garrett was named interim coach were by a combined seven points against the New Orleans Saints. Philadelphia Eagles and Arizona Cardinals.
(Reporting by Gene Cherry in Salvo, North Carolina. Editing by Frank Pingue)