He held the positions of Regents professor emeritus and writer-in-residence at St. John's University in Collegeville, Minnesota.
He spent his childhood and teen years in the Minnesota towns of Staples and Plainview, where he graduated from high school.
[2] Much of Hassler's fiction involves characters struggling with transitions in their lives or searching for a central purpose.
Hassler's novels have several recurring characters, including Miles Pruitt (the protagonist in Staggerford, who is referred to in A Green Journey, The Love Hunter, and The New Woman); Agatha McGee (in Staggerford, A Green Journey, Dear James, The Staggerford Flood, and The New Woman); Larry Quinn (in The Love Hunter and Rookery Blues); and Frank Healy (in North of Hope and The New Woman).
In 1990, Hassler's novel A Green Journey was adapted as a television movie, The Love She Sought, starring Angela Lansbury and Denholm Elliott.