The youngest of four siblings, Thornton began playing soccer at age five and lacrosse a year later. His father Ernest, who had endured his share of injuries while playing football at Kentucky State College, had gently steered him away from the gridiron sport. Thornton was a three-sport star in soccer, lacrosse and basketball at The John Carroll School. He attended Essex Community College before completing his education at Loyola College in Maryland. At Loyola, he played both soccer and lacrosse, the latter as a third-team All-American.
Playing career
Professional
Thornton was selected in the 7th round by the New York/New Jersey MetroStars in the 1996 MLS Inaugural Player Draft. With then-former US national keeper and local star Tony Meola entrenched in Metro nets, Thornton played only six games during his first two seasons in MLS. In 1997, he played a few games on loan with the North Jersey Imperials of the USISL, whose starting goalie was future Metro and US national teamer Tim Howard. The Chicago Fire took Thornton in the 1997 MLS Expansion Draft, and his career took off. Beating out Mexican international Jorge Campos for the starting job, Zach lead the expansion Fire to that year's MLS Cup, and was named the league's Goalkeeper of the Year. He spent six years with the Fire, an automatic starter throughout his time there, and frequently at the top of MLS goalkeeping charts. In 2004, Thornton signed a six-month contract with Portuguese club Benfica. He didn't get a single first-team match with them, however, and re-signed with the Fire late in the MLS season. Thornton did not get any playing time, as his former backup Henry Ring had taken hold of the position in Thornton's absence. In 2005 Thornton would reclaim his spot in the Fire's net missing only a few matches due to injury. Thornton was traded to Colorado Rapids on March 2, 2007 but was waived at the end of the year. He was subsequently picked up by New York Red Bulls as the team's reserve goalkeeper for 2008. Thornton was traded to Chivas USA on August 2, 2008. He and enjoyed an impressive first two seasons with the team, ultimately winning the 2009 Goalkeeper of the Year Award. Thornton was injured during the 2011 season with Chivas USA and lost his starting role. At season's end, the club declined his 2012 contract option and he entered the 2011 MLS Re-Entry Draft. Thornton was not selected in the draft and became a free agent.
International
Thornton's first cap with the US national team came in 1994 in a friendly against Jamaica. He backed up Kasey Keller at the 1996 Summer Olympics. Thornton wasn't able to translate his success in MLS to the international level, collecting only eight caps. On December 22, 2009, Thornton received another call up to play a friendly against Honduras on January 23.
Coaching career
Thornton was hired as a part-time volunteer goalkeeping coach for the Loyola Greyhounds men's soccer team on July 5, 2012. He was named to a similar position on a full-time basis for both the men's and women's programs at Elizabethtown College just over six weeks later on August 20. In 2015 Thornton was hired as Director of Goalkeeping for MLS franchise D.C. United.
