He becomes the fifth head coach in program history, succeeding Adrienne Carr, who stepped down in September after 15 years at FDU.

Mager has served as the Head Varsity Girls Lacrosse Coach at Morristown High School since 2011, recording over 100 career victories. The team won the 2018 Morris County Championship, finishing ranked No. 8 in New Jersey and was one of the top-125 teams in the country. Morristown HS advanced to the semifinals of the county tournament in 2015, 2016, and 2017 and his 2017 team finished the year ranked No. 9 in the state. The team was ranked as high as No. 3 in the state and was in the top-20 nationally.

During his tenure at Morristown, he coached five All-Americans, 17 Academic All-Americans, 6 All-State players plus a top-10 Player of the Year Finalist. 10 players were named to the Senior All-Star team, while five were chosen to Under Armour Underclass Team. Under Mager's watch, nine student-athletes were recruited to play Division I lacrosse; two to play division two and 7 to play Division III. Three players have won National Championships at Division I, II and Division III. Through various clubs, he has worked with over 100 student athletes through the recruiting process. He was honored by MUGS Media as the Girls Lacrosse in 2014 and 2017, while his 2017 Morristown team was tabbed as the MUGS Media Team of the Year. The Daily Record honored Mager as the Girls Lacrosse Coach of the Year in 2013.

In February 2019, Mager was named the Director of Recruiting and Events for All Lax Lacrosse after previously holding the role of Director of Lacrosse Operations and National Director of Recruiting at Mad Dog Lacrosse from January 2018 until January 2019. Additionally, Mager held the position of Director of Recruiting at Tri-State Girls Lacrosse in the year prior. Mager is the President of the New Jersey North Coaches Association and is the Chair Person for the All-American North Selection Committee. He served as an Assistant Women's Lacrosse Coach at Manhattan College in the fall of 2011, handling eligibility and recruiting. Mager also was the Assistant Varsity/JV Hockey Coach at Morristown High School.