Professional skills, also known as non-technical, transferable or 21st century skills are recognized worldwide as very important for university graduates. These skills include communication, problem-solving and the ability to function successfully on a multi-disciplinary team. While academic programs worldwide strive to develop a solid professional skills base in students, these skills are notoriously challenging to teach and assess. This paper presents the Computing Professional Skills Assessment (CPSA), a performance assessment of the professional skills learning outcomes identified by the Accreditation Board for Engineering and Technology (ABET), for the computing discipline. The assessment consists of a performance task, a rubric and an implementation method. The CPSA has been developed in an iterative manner and results suggest that it can accurately and consistently elicit and measure the targeted professional skills in computing students within appropriate contexts. Considering two components of the Communities of Inquiry model, cognitive and social presence, specific reliability and validity protocols were developed for the CPSA. These involved a pilot study, iterative development and assessment of the rubric, establishment of interrater reliability, the production of computing specific scenarios which are suitable for non-native English language speakers, and an implementation strategy using asynchronous discussion boards. Based on our findings through more than 50 CPSA implementations over a sevenyear period, we have found that the instrument can be used reliably and validly as a measure of the professional skills learning outcomes for English as a second language learners in a computing program.