David Hernandez, a right-handed pitcher, began his Major League Baseball career with the Baltimore Orioles. Drafted by the Orioles in the 16th round of the 2005 MLB Draft, Hernandez worked his way through the minor leagues, showcasing his skills as a starting pitcher. He made his MLB debut on May 28, 2009, against the Detroit Tigers, where he pitched well, earning his first win.

Hernandez's time with the Orioles saw him transition from a starting pitcher to a relief role. In 2010, he was traded to the Arizona Diamondbacks along with Kam Mickolio in exchange for Mark Reynolds. This trade marked a turning point in his career as he became a full-time relief pitcher with the Diamondbacks.

During his tenure with Arizona, Hernandez became an integral part of their bullpen. In 2011, he recorded a career-high 11 saves and posted a 3.38 ERA over 69 games. His ability to strike out batters was a notable aspect of his pitching, as he consistently maintained a high strikeout rate. The 2012 season saw him continue to perform effectively, appearing in 72 games with a 2.50 ERA, further solidifying his role as a reliable reliever.

However, Hernandez faced challenges in 2014 when he underwent Tommy John surgery, causing him to miss the entire season. He returned to the Diamondbacks in 2015, but his performance was inconsistent as he worked to regain his pre-surgery form. Despite these struggles, he remained a valuable asset in the bullpen.

In 2016, Hernandez signed a one-year contract with the Philadelphia Phillies. His stint with the Phillies was short-lived, as he was traded to the Los Angeles Angels in August of the same year. With the Angels, he found some stability, appearing in 70 games and posting a 3.84 ERA.

Hernandez's journey continued as he signed a minor league contract with the San Francisco Giants in 2017, though he did not make an appearance for the team. Later that year, he returned to the Diamondbacks after being traded from the Atlanta Braves, who had acquired him earlier in the season. His second stint with Arizona was brief, as he was traded to the Cincinnati Reds in 2018.

With the Reds, Hernandez experienced a resurgence. In the 2018 season, he appeared in 57 games, achieving a 2.53 ERA and proving to be a dependable option out of the bullpen. The following year, he continued to pitch for the Reds, though his performance saw some decline, with a 4.56 ERA over 47 games.