"After more than eight years as CEO of this great club, now feels like the absolute perfect time to start the process of bringing in a new CEO to take Port Adelaide forward beyond 2020," Thomas said.
"I have been in discussion with chairman David Koch for some time and we have both agreed this is the best outcome for me personally and, most importantly, for the football club.
"Well run football clubs like Port Adelaide do not rush into transitions as important as this. Hence why we have given ourselves plenty of time to make this change in a steady, orderly manner."