He signed with the Giants and made his MLB debut in 2009, posting a 1.04 earned run average (ERA) in 11 games.

An injured lat muscle cost him the first two months of 2012, and he only pitched in six games for the Giants in the 2012 season, when they won their second World Series during his time with the club.

Runzler returned to the United States in 2015, pitching in the minor leagues with various organizations until the Pirates added him to their roster in September 2017.

[4][5] On August 1, he entered a game against the Chatham Athletics in the sixth inning with the bases loaded, two outs, and the Kettleers clinging to a 1–0 lead.

Runzler struck out Doug Pickens, ending the threat and preserving the lead in an eventual 2–1 victory that clinched the Western Division title for Cotuit.

In 15 games, he had a 1–2 record, a 3.44 earned run average (ERA), 24 strikeouts, six walks, and 15 hits allowed in 18+1⁄3 innings pitched.

He also appeared in one game for the Single-A short season Salem-Keizer Volcanoes of the Northwest League, allowing one run in one inning of work.

[8] In 2008, Runzler started the season with the Single-A Augusta Greenjackets of the South Atlantic League (SAL) but struggled.

[9] In 20 games, he had an 0–1 record, a 5.47 ERA, 26 strikeouts, 19 walks, and 25 hits allowed in 24+2⁄3 innings before getting demoted to Salem-Keizer in midseason.

[9] He earned a spot on the SAL mid-season All-Star team but did not participate in the game because he was promoted to the Single-A advanced San Jose Giants in the California League on June 8.

[2] He did not allow a run in his first 15 games with San Jose; combined with his last appearances with Augusta, Runzler threw 32+2⁄3 consecutive scoreless innings.

[2] Runzler pitched in 19 games with San Jose before moving up to the Double-A Connecticut Defenders in the Eastern League.

After seven appearances for them, he was promoted to the Triple-A Fresno Grizzlies in the Pacific Coast League (PCL), with whom he threw two scoreless innings before being called up to the Giants in September when rosters expanded.

[9] On September 4, 2009, Runzler made his MLB debut against the Milwaukee Brewers, striking out center fielder Jody Gerut on three pitches.

[9] Ranked the fifth-best prospect in the Giants organization by Baseball America in 2010, Runzler made the Opening Day roster for the first time in his career.

[2] He got his first major league win on May 4, pitching a scoreless 11th inning and striking out three in a 12-inning, 9–6 victory over the Florida Marlins.

[12] In the July 8 contest, against the Brewers, Runzler entered the game with one out in the sixth, the bases loaded, and San Francisco clinging to a 6–3 lead.

[16][17] While he was in Fresno, the Giants used him as a starting pitcher, though sportswriter Andrew Baggarly thought the move was a temporary one meant to get him more time on the mound.

[22] Both pitchers were sent to Fresno on August 28 to make room for Romo and Andrés Torres to return from the disabled list,[23] but they were recalled on September 6 after rosters had expanded.

[16] Runzler exited the final game of the season on September 28 with what was later diagnosed as a torn latissimus dorsi muscle.

[1] After aggravating his lat muscle injury during the first week of spring training in 2012, Runzler began the season on the disabled list.

[2] He rehabilitated in extended spring training and with San Jose before being activated from the disabled list and assigned to Fresno on June 27.

[8] An unlikely candidate to make the Giants roster in 2013, as San Francisco had three other left-handed relievers, Runzler was optioned to Fresno on March 24.

[2] Runzler signed a minor league contract with the Minnesota Twins that included an invitation to spring training on November 4, 2015.

[37] In eight games for Pittsburgh, Runzler had no record, a 4.50 ERA, four strikeouts, two walks, and seven hits allowed in four innings pitched.

[43] On April 9, 2021, Runzler was hired as the pitching coach for the Milwaukee Milkmen of the American Association of Independent Professional Baseball.