Professor Flashman preferred to follow a career in mathematics. He had taught at Mount Holyoke College, Baruch College of CUNY, and Bard College before spending over 35 years mainly at Humboldt State University(HSU) where he is now retired as Professor Emeritus of Mathematics. At HSU was a Pre-Law advisor  for many years. Since fall, 2017, he has been a visiting scholar at the University of Arizona.

For too many years Professor Flashman has been working on a draft of the Sensible Calculus Book. He was book and media editor for UME TRENDS in which he wrote the column "Noteworthy Books and Such" until it ceased publication in 1996, and was a member the MAA committee on Calculus Reform And The First Two Years (CRAFTY) for 6 years ending in January, 1999. He was chair of the MAA Special Interest Group in the Philosophy of Mathematics from 2007 to 2009. In summer 2013 he started work on a resource for mapping diagrams: Mapping Diagrams from A(lgebra) to C(alculus) and D(ifferential) E(quation)s. A Reference and Resource Book on Function Visualizations Using Mapping Diagrams which is available on the internet in a working draft version using GeoGebra. He has made numerous presentations at local, national, and international conferences on his work visualizing functions with mapping diagrams.

His main hobbies currently revolve around music and trying to be on time.