In 1977 she became project manager in software development at Lawrence Berkeley Laboratory, where she guided the development of a new Integrated Control System for the Bay Area Rapid Transit system.

In 1985 together they founded the software development firm Project Technology Inc. Shlaer was also a Fellows of the Association for Computing Machinery.

Shlaer started her software engineering career at Los Alamos National Laboratory as a programmer.

The existing control system software was considered impossible to continue using, making replacement necessary.

[2] In the developing of a new control system for the Bay Area Rapid Transit, Shlaer and Mellor sought to regulate mechanisms of software development and began to design new methods of project management.