Manzoor Alam, 73, said he is “privileged” to have taken the helm of Sanquhar post office in Dumfries and Galloway, which was established in 1712.
The Birmingham stamp collector and postal historian moved to secure the future of the shop after it was put up for sale.
He took up the appointment on July 3 and the post office has undergone a refurbishment before reopening today.
Mr Alam, who will run the shop with his wife and daughter, also hopes to open a postal museum in the village in future.
He said: “I feel really very pleased, very privileged to have a post office with such a rich heritage.”
Sanquhar post office began as a staging post for mail carriages more than 300 years ago and has been in continuous service ever since.
Eight years older than its nearest rival in Stockholm, Sweden, it is officially recognised by both the Universal Postal Union and Guinness World Records.
The branch is a popular attraction for postal and stamp enthusiasts, who visit to have their letters marked with a hand-stamp bearing the legend: “The World’s Oldest Post Office”.
Mr Alam is only the 16th person to run the branch and takes over from postmistress Penny Murphy.
He acted after reading in the monthly newsletter of the Edinburgh Philatelic Society that its future was in doubt.
There were fears it could close after it was put on the market and a possible community takeover did not materialise.
The listed price was £195,000 for the post office and adjoining accommodation and the advert offered a salary package of around £50,000 a year.
Mr Alam had no ties to the Royal Burgh of Sanquhar but hopes the two local staff members he employs will help him get to know the community, and vice versa.
He said: “It was July last year when I saw mention of Sanquhar post office in the bulletin and I realised it was going to be lost, that it was going to be attached to a supermarket or something, and I thought if that happened the heritage would be lost.
“I thought I should do something about it.
“There’s nothing much to show it is the world’s oldest post office and when visitors come they have an expectation. So I will be going and knocking on doors to try to establish a museum too.
“If it happens it will be the first postal museum in Scotland.
“I also have plans to set up a philatelic society in Sanquhar.”
The year the post office was established saw the births of Frederick the Great, philosopher Jean Jacques Rousseau and British prime minister George Grenville.
The Act of Union between Scotland and England was sealed five years earlier in 1707.