Ashton, who made his debut for The Addicks against Carlisle United in the Carling Cup last September, is primarily a left-back and could help strengthen the left flank for The Lions in the run-in to the end of the season.
"Nathan looks a good prospect," Donachie told the club's official site. "We've taken him now with a view to something longer term perhaps. He'll be a useful addition to the squad."
Ashton's signing could spell the end of Tony Craig's Lions career after a number of poor performance at the back for Millwall this season.