McLouth's shot came as he was leading off the first inning and the O's never trailed after that.
Jim Thome provided a pair of run-scoring singles in Baltimore's fourth straight victory, which allowed the O's to hang onto at least a share of the American League East league.
Joe Saunders (3-3) earned the win with 7 1/3 solid innings, yielding three runs on eight hits with no walks and five strikeouts.
Bosox starter Zach Stewart (1-4) was touched for for five runs on seven hits in just 2 2/3 innings.
Daniel Nava and Cody Ross homered as the Red Sox reached 90 losses for the first time since 1966.