The Cardinals have gone 96-74 under Hartnett since the school's first season in 2010, including three-straight trips to the NCAA Division III Tournament (2013-15) and back-to-back Ohio Athletic Conference (OAC) Tournament titles in 2014 and 2015. His squad also earned the 2014 OAC regular-season championship, and made the title game in 2016.

Hartnett has seven winning seasons under his belt, capturing the inaugural OAC Tournament crown in 2014 with a 13-8 win over John Carroll. Otterbein has had 44 all-conference players under his guidance, including standouts Marcus Willis and Robbie Guiliano going on to sign professionally with the Ohio Machine. Mikey O'Neal (2014) and Joe Sanfillipo (2019) received OAC Clyde Lamb Awards, presented annually to the top male and female student-athletes at each of the 10 league schools.

Hartnett came to Otterbein from St. John Fisher College in Pittsford, N.Y. where he had been the assistant coach for the men’s lacrosse team since graduating from the College of Wooster in 2003. In his five years at St. John Fisher, Hartnett helped build a team from the ground up and take it to the Eastern College Athletic Conference (ECAC) playoffs three years in a row and to a school best 11-3 record in 2008.
He served as the offensive coordinator in 2004 and 2005 at St. John Fisher, and as the defensive coordinator since 2006. While offensive coordinator, the team finished the 2005 season averaging 10.8 goals a game, an increase over the 8.3 average in 2004. As defensive coordinator, the unit improved to 28th of 130 teams in the country, allowing just 7.3 goals an outing in 2008, a sharp drop from the 13.4 average in 2004.

A native of Canandaigua, N.Y, Hartnett received his Bachelor of Arts in History from Wooster in 2003. He earned four varsity letters in lacrosse, finishing as the school’s fourth all-time leading scorer. As a freshman, Hartnett scored 52 points and was runner-up for the North Coast Athletic Conference (NCAC) Newcomer of the Year award. He was a 2x member of the All-NCAC team and served as captain his junior season.

Harnett and his wife, Hilary, now reside in Westerville.