Gmachl earned her M.Sc. in Physics from the University of Innsbruck in 1991. She went on to receive her Ph.D. in Electrical Engineering from the Technical University of Vienna in 1995, graduating sub auspiciis Praesidentis (with special honors by the president of the Austrian republic). Her studies focused on integrated optical modulators and tunable surface-emitting lasers in the near infrared. From 1996 to 1998, she was a Post-Doctoral Member of Technical Staff at Bell Laboratories. In 1998, she became a formal Member of Technical Staff at Bell Labs and in 2002 she was named a Distinguished Member of Technical Staff, in part due to her work on the development of the quantum cascade laser. In 2003, she left Bell Labs and took a position as Associate Professor in the Department of Electrical Engineering at Princeton University, where she is currently working as a full Professor since 2007.