The 34-year-old former Scotland international has scored 185 goals in a five-club career, which has included three spells with Peterborough United.
He has made 25 appearances this season and scored three goals, but has only started 13 league games.
Mackail-Smith will immediately go into the squad to face Lincoln City at Meadow Lane this weekend.
“Craig has built a very good reputation in the game,” said Neal Ardley.
“In this window we wanted to add players who know how to excel at this level, along with a couple of exciting young prospects. We wanted that mix of experience and quality.
“He makes me feel tired just watching him play. He doesn’t stop running – he’s still got that yard of pace he’s always had and he’s got a lot of know-how and experience.
“Adding Craig on top of Mitch Rose is great and we’re hopeful of possibly adding another couple in the next day or so.”
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