It is hard to know how much of this ageing process has been natural, and how much has been due to the draining task of seeking political agreement and peace in Ireland.
If the process has had an effect on him, he has certainly affected its progress, by deploying his formidable political and diplomatic skills to keep it afloat through some rocky moments. Next week's final stretch of negotiations will provide the ultimate test of his talents.
The likelihood is that he and his staff will rise to the challenge, for he is, by almost universal consent, one of the most consummate political operators ever seen in Belfast. He is, as one admirer put it, a class act.
Mitchell was not born with a silver spoon in his mouth but he has built an extraordinary career, becoming one of the US's top public figures.
He was by turns an army intelligence officer, a trial attorney and a federal judge. A liberal Democrat, he spent 14 years as a US senator. He was majority leader in the senate, where for six consecutive years his peers voted him the most respected member.
After retiring from the senate, he turned down a nomination to the supreme court. But then Bill Clinton, a political ally, asked him to become involved in Northern Ireland. That was in 1994, he took what looked to be an innocuous post, agreeing to serve as Clinton's "economic envoy" to Northern Ireland.
The job was supposed to last for four months, but Mitchell has remained embroiled in the tortuous peace process ever since. With direct access to Clinton, Tony Blair and Bertie Ahern, he has come to play a central role.Reuse content