Vidal Nuno (1-1) came on to pitch for the Yankees starting the inning and McLouth greeted him by bashing a homer to right-center field for the decisive blow.
The blast handed the victory to closer Jim Johnson (2-4), who tossed a scoreless top of the 10th after blowing saves in each of his previous three outings, including Monday's 6-4 loss to New York.
Chris Dickerson went 3-for-4 with two solo homers for the Orioles, who evened their three-game series.
Travis Hafner drove in both Yankees runs.
NBC reportedly holds celebs hostage to Jimmy Fallon's show
Ray Liotta sues skin care company over use of likeness