He played college football for Northern Illinois University, where he was the Huskies' starting quarterback for four years.

He holds multiple Norwell High School records, including career all-purpose yards (7,103).

As a senior, he led the Norwell Knights to a 14–1 record and the runner-up spot in the Indiana 3A state championship.

He was also a conference, sectional, and regional discus champion who finished sixth in the Indiana State Finals as a junior, with a top throw of 52.71 meters.

[4] Regarded as a two-star recruit by Rivals.com, Harnish's only scholarship offer came from Northern Illinois, which he accepted.

[5] Sources: On August 6, 2006, Harnish committed to Northern Illinois University (NIU),[6] the only FBS school that made him a scholarship offer.

[7] Despite not being heavily recruited out of high school, Harnish became one of the greatest quarterbacks in NIU history.

He competed for the backup job against Matt Hasselbeck in the 2013 preseason, but was waived and later signed to the practice squad.

[27] On September 29, 2014, Harnish signed with the Minnesota Vikings, who placed him on their practice squad after starting quarterback Matt Cassel was placed on injured reserve and Teddy Bridgewater had sprained his ankle.

[30] On October 7, 2014, Harnish was re-signed to the Vikings practice squad and quarterback McLeod Bethel-Thompson was released.