Hearn got her ticket to join the 410-strong Australia team heading for Brazil when Kelly Layne withdrew from the team over the weekend because her horse was suffering from an injury.
"LLOYDY and I are off to Rio! Thank you everyone for your support!" Hearn posted on her Facebook page after getting the news that she had been named to the dressage team with her warmblood gelding (Lloyd) Remmington.
Hearn will be the second oldest Australian to make their Olympic debut after sailor Harold Brooke, who was 61 when he got his first taste of the Games in Rome in 1960.
Although Hearn will bring a measure of maturity to an Australian Olympic team that has an average age of 26, she will not even be the oldest member of the dressage squad.
Her team mate Mary Hanna, one of three Australians who will be taking part in their fifth Olympics, takes that honour at the age of 61.
The youngest athlete in the 207-man, 203-woman Australia team is 16-year-old footballer Ellie Carpenter, who was a babe in arms when Australia hosted the Games in Sydney in 2000.
Australia sent its biggest ever team of 632 athletes to the Sydney Games and the Rio delegation is the equal fifth largest, matching the party that was assembled for London four years ago.
The tallest member of the team is NBA basketballer Andrew Bogut at 213cm (7ft), while Beijing silver medallist in diving Melissa Wu is the shortest at 153cm.
Australia will compete in 26 sports in Rio - they have no entries for fencing or handball - and have targetted a return to the top five on the medal table after finishing eighth in London.
(Reporting by Nick Mulvenney, editing by Sudipto Ganguly)
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