Arleen Cassidy was Trudy Monk (nee Ellison)'s best friend in high school. They both attended Ashton Preparatory School, where, according to her, Trudy would help Arleen through English Lit, and Arleen would help Trudy through math.

Arleen later became headmaster of the school, and hired Adrian Monk to investigate the mysterious death of one of her teachers, Beth Landau.