The Grade II listed building will serve its last customers on Wednesday with the new branch opening on Thursday morning.
Paddy Mellon, Frenchgate Shopping Centre general manager, said: “We have a brilliant range of retailers and services at the centre and we are really pleased for the Post Office to become a part of our fantastic mix as it will enhance and greatly strengthen our diverse offering.
“We are thrilled that the Post Office have been able to move the premises into WHSmith, maintaining vital post office services in the town. Plus, the same team who head up the branch will also be coming along to run the new addition, so we are looking forward to welcoming a few new faces to the centre.”
Roger Gale, Post Office sales and trade marketing director, said: “The move will help to ensure that the branch is commercially viable in the future and in a more central location which will protect services on which customers in Doncaster depend.
“We know that the best way to provide a great service for customers is to evolve our business and adapt to their changing needs. We know that our customers never stop changing, so neither will we.”
The shock move was first announced in February.
The Grade II listed Priory Place building was built in 1885 by local builder Frederick Masters and has been home to Doncaster's main Post Office for decades.
The building is also thought to be built on top of the last resting place of a Scottish king who ruled more than 700 years ago.
Historians and archaeologists believe one time King of the Scots Edward Balliol could have his last resting place beneath the distinctive red brick building.
Balliol, who ruled north of the border from 1332-36, died in Doncaster in 1367.
Historian Peter Robinson said: “The current Post Office is built on top of a huge burial ground and at that time, that’s where a lot of eminent figures would have been laid to rest. At that time, the area was home to a large Carmelite friary and that’s one of the places where Edward could be buried.”
The new Post Office’s opening times will be follows: Monday 9am – 5.30pm, Tuesday 9.30am – 5.30pm, Wednesday to Friday 9am – 5.30pm and Saturdays 9am until 4pm.