Integrated Support Center for Nuclear Nonproliferation and Nuclear Security (ISCN) of Japan Atomic Energy Agency (JAEA), in cooperation with the International Atomic Energy Agency (IAEA), have developed a new Mock-complementary access (Mock-CA) exercise for online training. This Mock-CA exercise is a combination of a virtual tour of the JAEA’s research laboratory, a video taken of a Mock-CA demonstration, and power point presentations. JAEA successfully executed the exercise during its online regional training course on the State System of Accounting for and Control of Nuclear Material (Online RTC-SSAC) from 28 November to 10 December 2021. The Mock-CA exercise was developed based on the exercise that JAEA delivered for the IAEA International Training Course on safeguards for States with the Small Quantities Protocol (SQP) during the course in 2019, which was conducted in-person. The exercise scenario used for the SQP course was demonstrated at a JAEA research laboratory. This scenario was updated for the Mock-CA exercise by jointly working with the IAEA so that it covers the main CA processes, including the roles and responsibilities of the CA participants, notification to the state, access to the facility, environmental sampling, managed access, the relationship with the articles of Additional Protocol, and a follow-up activity. We have found that the developed Mock CA exercise is not only a powerful tool for online training, but also beneficial for in-person training. Addressing efforts to mitigate the affects of the COVID-19 pandemic has created a new tool for IAEA safeguards-related training that is beneficial even in the post-pandemic era. This paper describes the Mock-CA exercise development, implementation and analysis of the feedback from the participants.