Christian attended Borel Middle School in San Mateo and then attended Aragon High School in San Mateo, California from 1995–98, where he played varsity baseball all four years for John Arents and later Mike Trimble. He began his college baseball career with two years at Skyline College where he was All-Conference for both years. He took the scholarship he had earned to Auburn University in Auburn, Alabama, but transferred after missing most of the 2002 season with a torn labrum to Southeast Missouri State University, where he became an All-American and was voted "Newcomer of the Year." At Skyline he was a shortstop, but was moved to second base at Auburn. He was mostly an infielder at SEMO as well.
After going undrafted in the 2003 Major League Baseball Draft, he signed with the River City Rascals of the Frontier League, winning the starting second base job and playing 30 games during the 2003 season. In the fall of 2003, he went back to Auburn as a student assistant coach while trying to set up scout workouts; he set up several of these but was unsuccessful in getting signed. He returned to the Rascals for the 2004 season, where he hit.450 with 26 steals in 30 games.
The New York Yankees signed him sight unseen. In 2004, he played on the GCL Yankees and the Staten Island Yankees. He started the 2005 season with the Charleston RiverDogs, where he hit.290 and was promoted to the Tampa Yankees where he hit.306. He ended the season with a combined 55 stolen bases. In, Christian hit.276 with 6 home runs, 43 RBI and 68 stolen bases, 3rd in all of the minor leagues. He started the season with the Double-A Trenton Thunder, batting.235 with 3 home runs, 32 RBI and 18 stolen bases. On June 22, 2007, he was called up to Triple-A Scranton/Wilkes-Barre where he hit.325, and.271 overall, for the year. On June 24,, he was promoted to the Yankees' major league roster preceding their interleague series with the Pittsburgh Pirates. He started the same day, going two for four with a double and two RBI in his major league debut. On July 8, 2008, he stole his first base in the major leagues, stealing third base off Tampa Bay Rays relief pitcher Grant Balfour and catcher Dioner Navarro. The only two times he was caught stealing, he had fallen down between first and second base or had been leading far off first. Following the 2008 season he was non-tendered by the Yankees, making him a free agent.
Baltimore Orioles
He was signed to a minor league contract by the Baltimore Orioles on January 13,.
Second Stint with Yankees
On May 7, 2010 he was re-signed by the Yankees while playing for the Southern Maryland Blue Crabs in the Independent Atlantic League, and was assigned to their Double-A affiliate Trenton Thunder where he broke Kevin Thompson's franchise record for stolen bases and was re-promoted to Triple-A Scranton/Wilkes-Barre on September 6.
San Francisco Giants
On February 6, 2011, he signed a minor league contract with the reigning world champion San Francisco Giants. On September 6, 2011, the Giants purchased his contract and he was called up to the major league squad. Christian started 2012 with Triple-A Fresno, and ended up splitting the season with them and the Giants. With Fresno, he hit.343/.409/.508 with 7 HR, 35 RBI and 12 SB. With San Francisco, he hit.125/.197/.143 with 2 RBI and 2 SB. Justin won a World Series ring as a member of the 2012 Giants.
St. Louis Cardinals
On November 26, 2012, Christian signed a minor league deal with the St. Louis Cardinals with an invitation to spring training. He is projected to be starting outfielder for the Cardinals Triple-A Memphis Redbirds affiliate.
Tampa Bay Rays
Christian signed a minor league deal with the Tampa Bay Rays on January 14, 2014.
Quintana Roo Tigres
Christian also played with Tigres baseball team in Cancun, Mexico
