Meanwhile, two other ageing stars on the Giants' list, premiership players Luke Power and Dean Brogan, are yet to decide whether to go around again, while Stephen Clifton and Tim Segrave have been delisted.
After 251 matches McDonald was cut loose by the Demons at the end of 2010 by then coach Dean Bailey, who later conceded the decision had been a mistake.
And after initially appearing set to hang up the boots, the 35-year-old played amateur football for a year before being thrown a lifeline by Giants coach Kevin Sheedy.
But having played 13 matches for GWS in their debut season in the competition, McDonald has decided to concentrate on the assistant coaching role he has also been filling since joining the club.
"James' experience, leadership and on-field presence have been invaluable to our young playing group and he has shown everyone that he still had good football left in him," said Giants football manager Graeme Allan.
"We now look forward to him becoming a full-time assistant coach next year and further developing our young playing list."
McDonald's final match last Saturday was marred by an incident in which he bumped into the back of North Melbourne's Jamie Macmillan, drawing a one-match suspension, with Macmillan's coach Brad Scott abusing McDonald at quarter-time as a result.
Having now played 251 matches, 235 of them for Port Adelaide including the 2004 premiership, 32-year-old Cornes will play on for a 15th season.But triple premiership player Luke Power, who has played 302 matches, and 2004 Port premiership player Brogan, who has managed 183 appearances in 12 seasons, are yet to confirm their intentions.