In, he split his time between Low-A Hagerstown, and High-A Potomac, where, he combined to hit.281 with 27 home runs, 83 RBI and 35 stolen bases, the only player in Minor League Baseball to record at least 25 doubles, 25 home runs and 25 stolen bases in 2007." After the minor league season ended, the Nationals promoted Maxwell to the major leagues, though he "was told not to expect much from his first month in the majors." On September 5, he made his debut, as pinch hitter, and flew out to the warning track. Six days later, in his third pinch hitting appearance, Maxwell recorded his first career base hit, a pinch-hit grand slam off an 0–2 pitch from Chris Seddon, of the Florida Marlins. After the 2007 regular season, Maxwell played for the Peoria Javelinas in the Arizona Fall League. Maxwell was assigned to the Harrisburg Senators of the Double-A Eastern League in 2008 after hitting.280 in 13 spring training games for Washington, but missed most of the year with a fractured wrist suffered while diving for a flyball in the outfield. Maxwell spent most of the 2009 season with the AAA Syracuse Chiefs of the International League. He was called up late in the season because of injured center fielder Nyjer Morgan. On September 30, Maxwell hit a two-out full count walk off grand slam off of Francisco Rodriguez of the New York Mets on Fan Appreciation Day, the Nationals' last home game of the season. Maxwell split time with the AAA Syracuse Chiefs of the International League and the Nationals in 2010. On September 15, Maxwell hit a grand slam in the 2nd inning off of Mike Minor of the Atlanta Braves giving him three grand slams in four career at bats with the bases loaded. In September, Maxwell sat out two games with a minor thumb injury suffered on September 18. He returned September 21 against the Houston Astros. Maxwell underwent Tommy John surgery after the season.
On February 2, 2011 the Nationals traded Maxwell to the New York Yankees for minor league pitcher Adam Olbrychowski. Maxwell began the 2011 season with the Triple-AScranton/Wilkes-Barre Yankees, but his season ended prematurely due to a torn labrum. The Yankees designated Maxwell for assignment at the end of spring training in 2012.
Houston Astros
The Houston Astros claimed Maxwell off waivers on April 8. Maxwell led the team in home runs with 18. He batted.229 and had 53 RBIs.
Kansas City Royals
Maxwell was traded to the Kansas City Royals for Kyle Smith on July 31, 2013. He hit a walk-off grand slam on September 22, 2013 against the Texas Rangers. He was designated for assignment on May 16, 2014. After being re-added to the roster, he was designated for assignment again on June 30, 2014. Maxwell elected free agency in October 2014.
San Francisco Giants
Entering the 2015 spring training as a non-roster hopeful for the San Francisco Giants, Maxwell hit for power and a.300+ batting average, which earned him a contract for the 2015 season. He was designated for assignment on September 1, 2015.