Mead was activated to the major-league roster by the Red Sox following Sunday's 6-1 win over Toronto.
Mead will make his debut with Boston on Monday as the team begins a four-game series on the road against the Athletics. The 25-year-old is expected to play second base primarily with his new squad, a spot where he's made 35 major-league starts before. Mead has posted a career-high .254 average with 17 home runs, 48 RBI, 49 runs scored and six stolen bases over 284 at-bats in 87 contests with the Nationals this season, and he's batting .367 since the beginning of July. Tsung-Che Cheng was optioned to Triple-A Worcester in a corresponding move.