In the 2007 amateur entry draft, he was selected by the San Francisco Giants in the 9th round.
Minor leagues
In 2009, Runzler played for four different teams within the Giants farm system. He began the season with the Augusta Greenjackets, where he earned a spot on the South Atlantic League mid-season all star team. He was unable to attend the game because he was promoted to Class A San Jose in the California League. Runzler spent two months before moving on to AA and pitching for the Connecticut Defenders in the Eastern League. After two weeks, he was promoted to AAA Fresno in the Pacific Coast League before being called up to the Giants in September 2009. It is believed that Runzler is the only player in Giants' history to play minor league ball at all four full season levels and make the major leagues in the same season.
Major Leagues
On September 4, 2009, Runzler made his Major League debut against the Milwaukee Brewers, striking out center fielderJody Gerut on three pitches. Runzler appeared in 11 games with San Francisco in 2009, striking out 11 batters in eight innings with an ERA of 1.04. During the Giants' 2010 World Series season, he pitched in 41 games, going 3–0 with an ERA of 3.03. On May 28, 2011, Runzler was optioned to Triple-A Fresno to make room for Santiago Casilla, who was activated from the disabled list. On August 16, 2011 Runzler was recalled when Sergio Romo was placed on the 15-day disabled list. He was optioned to Triple-A Fresno on March 24, 2013. He was designated for assignment on September 3, 2013 and outrighted on September 6. He received an invite to the 2014 Spring Training but could not crack the major league roster. He was released on July 30, 2014.
Runzler signed a minor league contract with the Minnesota Twins in November 2015 with an invite to major league spring training. He became a free agent after the 2014 season.
On February 5, 2018, Runzler signed a minor league deal with the Tampa Bay Rays and then he was released on March 11.
Sugar Land Skeeters (second stint)
On March 29, 2018, Runzler signed with the Sugar Land Skeeters of the Atlantic League of Professional Baseball. He became a free agent following the 2018 season.
On February 1, 2019, Runzler signed a minor league deal with the Boston Red Sox; he was assigned to the Triple-A Pawtucket Red Sox. He was released on July 15, 2019.
Sugar Land Skeeters (third stint)
On July 22, 2019, Runzler signed with the Sugar Land Skeeters of the Atlantic League of Professional Baseball. He became a free agent following the season.
Sultanes de Monterrey
On February 18, 2020, Runzler signed with the Sultanes de Monterrey of the Mexican League.
Awards
Selected to South Atlantic League All-Star team representing Augusta Greenjackets in July 2009.
On November 19, 2011, Runzler married Michelle Phalen, a sports reporter for KSBW TV in Salinas. Originally from Scotts Valley, Michelle played tennis at Occidental College.