He played in Major League Baseball (MLB) for the New York Yankees, San Francisco Giants, Pittsburgh Pirates, and Cleveland Indians.

[4][5] The New York Yankees selected Kontos in the fifth round of the 2006 Major League Baseball draft after his junior year.

[6] With a 7–3 win–loss record, 2.64 ERA, and 82 strikeouts in 78 innings pitched, Baseball America named Kontos a Short-Season All-Star.

[9] After experiencing tightness in his pitching arm in June 2009, Kontos went on the disabled list in what he thought was a precautionary measure.

Magnetic resonance imaging diagnosed a torn ulnar collateral ligament in his elbow, which required Tommy John surgery, forcing him to miss the rest of the season.

[25] Kontos got his first major league win in relief of Barry Zito, successfully recording 7 outs without allowing a baserunner.

Kontos entered the postseason for a second straight season and won the 2012 World Series with the Giants in a four-game sweep of the Detroit Tigers.

[26] On June 11, 2013, Kontos was ejected for the first time in his Major League career after hitting Pittsburgh Pirates center fielder Andrew McCutchen with a pitch.

[28] On July 22, Kontos threw a career-high 63 pitches in an 11–0 loss to the Cincinnati Reds that helped save the Giants' bullpen before a doubleheader the following day.

On September 20, 2013, Kontos achieved notoriety as he surrendered Alex Rodriguez's 24th career grand slam that broke the record formerly held by Lou Gehrig.

[33] Kontos appeared in a career-high 73 games, recording a career-best 0.941 WHIP and holding opponents to a .159 average with runners in scoring position, third-best in the Majors.

[34] On February 1, 2016, Kontos agreed to a one-year, $1.15 million contract with the San Francisco Giants, avoiding arbitration.

[47] Kontos joined NBC Sports Bay Area as a San Francisco Giants pre- and postgame analyst for the 2020 season.

[48] Kontos' younger brother, Chris, played on a travelling baseball team, known as the Renegades, that was coached by Steve Bartman and his second cousin Tony.