They made the discovery during work to conserve Ta-Kr-Hb, believed to be a priestess or princess from Thebes, before she is displayed in the new City Hall Museum in Perth.
The mummy, which is nearly 3,000 years old, was in fragile condition after being targeted by grave robbers in ancient times.
Work was needed to ensure her condition did not deteriorate further.
Conservators were surprised to discover painted figures on both the internal and external bases of the coffin trough when they lifted her out.
Both figures are representations of the Egyptian goddess Amentet or Imentet, known as the She of the West or sometimes as Lady of the West.
Dr Mark Hall, collections officer at Perth Museum and Art Gallery, said: "It was a great surprise to see these paintings appear.
"We had never had a reason to lift the whole thing so high that we could see the underneath of the trough and had never lifted the mummy out before and didn't expect to see anything there.
"So to get a painting on both surfaces is a real bonus and gives us something extra special to share with visitors."
Conservators Helena and Richard Jaeschke have been working closely with Culture Perth and Kinross on the project, with work starting in late January.
Further research will be carried out on the paintings to find out more about the history of the mummy.
The painting on the interior base of the coffin trough was previously hidden by Ta-Kr-Hb and is the best preserved of the two.
It shows Imentet in profile, looking right and wearing her typical red dress.
Her arms are slightly outstretched and she is standing on a platform, indicating the depiction is of a holy statue or processional figure.
Usually, the platform is supported by a pole or column and one of these can be seen on the underside of the coffin trough.
The mummy was donated to Perth Museum by the Alloa Society of Natural Science and Archaeology in 1936.
It was presented to the society by a Mr William Bailey, who bought it from the curator of the Egyptian Museum in Cairo.
Dr Hall said: "The key thing we wanted to achieve was to stabilise the body so it didn't deteriorate any more so it has been rewrapped and then we wanted to stabilise the trough and upper part of the coffin which we've done."
He added: "Doing this means everybody gets to find out a lot more about her.
"One of the key things is just physically doing the work so we have a better idea of the episodes Ta-Kr-Hb went through in terms of grave robbers and later collectors in the Victorian times so we can explore these matters more fully and we can share that with the public."
Culture Perth and Kinross is campaigning to raise money for the conservation of Ta-Kr-Hb as she prepares to go on display at the City Hall Museum in 2022.It has a JustGiving page at https://www.justgiving.com/campaign/CPKmummy
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