That means McLouth passed his physical and that his $10.75 million deal is official.
The Nationals envision the veteran, a former All-Star and Gold Glove winner, as a perfect fourth outfielder, since he's able to play all positions, is a plus defender and adds a dimension of speed to the bench.
In order to make room for McLouth on the 40-man roster, the Nats designated outfielder Corey Brown for assignment.
Brown hit .254 with 19 home runs, 56 RBI, 12 stolen bases and 57 runs scored for Triple-A Syracuse this season. He appeared in 14 September games for the Nationals, hitting .167 (2-for-12) with one double, one homer and one RBI.
Brown was acquired by the Nats along with right-hander Henry Rodriguez in the trade that sent Josh Willingham to the Athletics. With McLouth's signing, Brown was all but boxed out of a roster spot.