He attended South Point High School in Belmont, North Carolina, where he played trombone and lettered in baseball and was all-conference and all-state in 2003 and 2004.

Having ended up at the Academy by chance, a Navy football recruiter named Buddy Green watched him pitch a bullpen session in high school.

Green found out that Harris was not interested in football, but was open to attending the Naval Academy, so he made phone calls on his behalf.

[2] In spite of the path that led to his entrance into Navy, Harris spent his freshman year as a corner infielder and utility player.

[3] However, when incoming coach Paul Kostacopoulos watched Harris throw during a practice session after his freshman season, he asked him to pitch in the bullpen, where he reached 91 miles per hour (146 km/h).

[3] As a sophomore, he claimed the Patriot League pitching triple crown with a 10–3 record, 1.74 earned run average (ERA) and 113 strikeouts (SO) in 13 games started.

[1] One of his teammates at Navy, Oliver Drake, also a pitcher, was a selection in the same amateur draft, and made his major league debut for the Baltimore Orioles the month following Harris.

[6] When he graduated, he was one of the highest-rated pitching prospects for the amateur draft, with a fastball that could reach 95 miles per hour (153 km/h) and a cutter with the ability to break bats.

[7] Following graduation in 2008 from the Naval Academy, Harris served in the United States Navy for four years, eight months and eight days, attaining the rank of Lieutenant (LT).

[3] LT Harris then contributed to a diplomatic mission to Russia on the USS Carr, an Oliver Hazard Perry-class frigate, and to South America for anti-drug operations.

[12][13][14][15] He twice petitioned for early release from active duty Navy to the reserves to commence his professional baseball career,[16] in the manner of NBA legend David Robinson.

[7] John Abbamondi, a former Naval flight officer and the Cardinals' assistant general manager while Harris was in the Navy, wrote letters on his behalf for the early release.

[20] Harris made his big-league pitching debut at Miller Park in Milwaukee against the Brewers on Saturday, April 25, in relief of starter Adam Wainwright, who had injured himself as he ran to first base on a pop out while batting.

[29] An arm soreness delaying Harris' spring training preparation, the Cardinals placed him on the disabled list prior to the start of the 2016 season due to nerve compression syndrome.