Johnstone played field hockey for four years at Maine, where she garnered many honors. As a senior, Johnstone helped to lead the 18th-ranked Black Bears to the America East Conference Championship, earning Second Team NFHCA Northeast Regional All-America Honors. A two-time First Team All-Conference selection, Johnstone was also a Second Team Regional All- American in 2000 and a First Team Regional All-American in 2001. The leading scorer for Maine in both 2000 (19 points) and 2001 (27 points), Johnstone's 27 points in 2001 ranked second in the America East.
This past summer, Johnstone was a Futures Head Coach for Elite Site in Connecticut and Rhode Island, working at several camps including Champions Edge, Elite Field Hockey Camp, Harvard Field Hockey Camp, and Dartmouth Elite. During the summer of 2004, Johnstone was a Futures Head Coach for Site 4 State of New York, after serving as a Futures Assistant Coach for Maine site in 2002. Johnstone has also coached at the Cornell Field Hockey Camps (Camp Director, 2003), the Maine Camp (Head Coach, 2002-2003; Assistant Coach, 2001); and the Temple Clinic (Instructor, 2000-2001).
A native of Dallas, Pennsylvania, Johnstone received a bachelor's degree in Secondary Education with a concentration in Mathematics from Maine in 2003. A Dean's List student, Johnstone earned the Dean Smith Award in 2001 and the Presidential Academic Achievement Award in 2002.
Johnstone continues to play field hockey for the US Field Hockey EPTC Team, and represented the New England Region as a member of the New England Fire, which competed in a tournament in Virginia Beach. She resides in Cranston, RI.