He has previously played in Major League Baseball (MLB) for the Oakland Athletics, Kansas City Royals, Pittsburgh Pirates, Miami Marlins, and San Francisco Giants.

[3] Mazzaro was traded along with minor league pitcher Justin Marks to the Kansas City Royals for David DeJesus on November 10, 2010.

[5] Besides setting a new franchise record for most runs yielded in a game, he became the first pitcher in the modern era (since 1900) to allow that many without pitching 3 innings.

His 14 earned runs given up was also the most by a relief pitcher since Les McCrabb of the Philadelphia Athletics allowed the same amount in four innings in a 19–4 defeat to the Boston Red Sox on April 16, 1942.

[7] Mazzaro was called back to Kansas City on June 7 after Royals starting pitcher Sean O'Sullivan was placed on the disabled list.

[8] On November 20, 2012, the Royals designated Mazzaro for assignment as they cleared room on the 40-man roster ahead of the Rule 5 draft.

On April 30, 2016, Mazzaro was called to the major league roster in order to replace the struggling Mike Broadway.

[15] He made only 2 starts for the Triple–A Louisville Bats, allowing 13 runs (12 earned) on 13 hits and 3 walks with 3 strikeouts in 4+1⁄3 innings pitched.

[17] Mazzaro did not play in a game in for the team due to the cancellation of the 2020 ALPB season because of the COVID-19 pandemic and became a free agent after the year.