A 2005 graduate of Justin-Siena High School in Napa, California, Wallace earned four varsity baseball letters for head coach Allen Rossi. He helped lead Justin-Siena to a 97–9 record during his prep career, including a perfect 27–0 senior season. He broke nine school records during his prep career and holds single-season school records for walks, runs, hits, RBI, home runs and slugging percentage. Among his numerous accolades, Wallace was named a Collegiate Baseball/Louisville Slugger All-American and 2005 first-team All-State (CalHiSports.com) and 2005 California Small Schools Player of the Year. He was also a four-time All-Marin County Athletic League selection, 2004 and 2005 All-Napa County Offensive Player of the Year.
Wallace is a two-time Pac-10 Player of the Year (2007 & 2008) and a two-time Triple Crown Winner at Arizona State University. He was the 12th Sun Devil to win the Pac-10 Player of the Year award, and the first since Dustin Pedroia shared the award in 2003.
On July 29, 2010, he was traded again, this time to the Houston Astros, in exchange for Minor League outfielder Anthony Gose, who had just been obtained by the Astros from the Phillies as part of a trade for Roy Oswalt. Wallace was sent to the AAA affiliate, the Round Rock Express, for one day and did not appear in a game before being called up by the Astros on July 31, 2010 after Lance Berkman was traded to the New York Yankees. Wallace made his Major League debut against the Milwaukee Brewers on July 30 and went 0–4. The next day, he got his first career hit, a single, against Brewers' pitcher Randy Wolf. On September 7 Wallace hit his first career home run off Chicago Cubs pitcher Carlos Silva, tying the game.
Wallace started the 2012 season with the RedHawks, and was recalled by Houston on June 3 when Carlos Lee was placed on the disabled list. He returned to Oklahoma City on June 17 when Lee was re-activated. Wallace was recalled again on July 30 when Brian Bixler was optioned to Oklahoma City.
Wallace started the 2013 season with the Astros. However, due to a slow start in which he batted .042 in 17 games, he was optioned to Oklahoma City on April 18. On June 25, he was called back up to start against the St. Louis Cardinals.
On February 6, 2014, the Astros designated Wallace for assignment to make room for Jerome Williams. He was released on March 12.
On December 13, 2014, Wallace signed a minor-league contract including an invitation to major-league spring training with the San Diego Padres. On June 19, 2015, the Padres called Wallace up to the majors.