This spring season, his first with the Cowboys, Mertz appeared three times out of the bullpen and finished the year with a 2.45 ERA. Though he had no record in 2015, in 3 2/3 innings pitched he held opponents to a .143 average.

Before joining Oklahoma State as a junior, Mertz spent two seasons with Cisco College. Perfect Game USA ranked Mertz as the 18th JUCO player in the country. As a freshman, Mertz was 6-5 with a 3.77 ERA in 80 innings pitched, and was a JUCO All-Star. He continued his success in his sophomore year, notching a 2-1 record that included a complete game and a 2.87 ERA to go with 19 strikeouts in 15 2/3 innings, before a season-ending injury sidelined him.