During the late 1980s and early 1990s, Mays worked as a government economist in Washington specializing financial institutions, first at the Government Accountability Office, and later at the Office of Thrift Supervision. There she was part of a team of economists who built the first industry model to evaluate the interest rate risk profile of S&Ls in reaction to the savings and loan crisis of the 1980s and 1990s. During this time Mays also published, with Anthony G. Cornyn, Interest Rate Risk Models: Theory and Practice (1997) a collection of articles on interest rate risk measurement and management.
In 1996, Mays became involved with credit risk modeling when she went to work for Freddie Mac. In 1998 she published Credit Risk Modeling: Design and Application, and in 2001, the Handbook of Credit Scoring. Since 1998, Mays has been a banking economist, running modeling and analytics organizations first for Citi, then JP Morgan Chase. Her 2004 book, Credit Scoring for Risk Managers is a widely used reference book in the credit scoring arena.