Esposito (AUS) started the Combined Event in 4th place and overtook the top three with a superb exhibition of composure and stamina. Elodie Clouvel of France also reined in the leader, Oktawia Nowacka (POL), to claim silver and Nowacka (POL) held on to win bronze.
Gold was a truly remarkable achievement by 24-year-old Esposito (AUS), albeit that she finished 7th in the 2012 Olympic Games in London. Her best achievements to date were 3rd place in the 2015 World Cup Final and 3rd place in a World Cup stage in 2012. She had been struggling for much of this year with an Achilles injury.
"I still can't believe it. First person ever in Australia to win a medal in pentathlon, and I can't describe it," said a beaming Esposito (AUS). "I was just thinking of all the hard training I've put in in the past four years. It's been a tough road and especially this year, I had a few injuries."
Clouvel (FRA) added: "It was a very good day. My shooting was not so good, so I was just thinking 'run fast, run fast'. But I got the medal and it's fantastic.
"I dedicate the medal to my family, my boyfriend Valentin Belaud, and tomorrow I will be here yelling 'Allez Valentin, go go go'. I dedicate also to Alexis Vas(bronze-medallist boxer at Beijing 2008, who died in 2015)(FRA), that is for you."
When the athletes received their medals from Claudia Bokel, IOC Executive Board Member and Chair of the IOC Athletes Commission, It was a thrilling conclusion to a dramatic and historic day at the Deodoro Stadium, where IOC President Dr Thomas Bach was among the audience.
Nowacka (POL) established a 12-point lead after the Fencing Bonus Round and maintained that advantage during the Riding competition to start first in the Combined Event. And she continued to keep distance between herself and 2nd-placed Clouvel (FRA) until the 3rd lap, having demonstrated great poise on the shooting range. But she began to tire and was caught on the final range by Esposito (AUS).
As a strong runner, there was no way she was going to be caught after leaving the range first, and she ran past her amazed and delighted family and supporters in the stand before taking the tape.
Nowacka (POL) and second-placed Clouvel (FRA) each scored 293 in Riding, picking up only one penalty, while Melanie McCann (CAN) had a clean round to give herself a chance in the Combined Event with a 29-second handicap. Esposito (AUS), Chen Qian (CHN) and Natalya Coyle (IRL) were also within 45 seconds of Nowacka (POL).
The Riding competition was truly dramatic, with a number of athletes incurring scores of zero due to multiple refusals or falls. Incredibly, both of the former Olympic champions suffered this fate: first Laura Asadauskaite (LTU) and then Lena Schoeneborn (GER). As a result they began the Combined Event with a 2-minute handicap, although Asadauskaite (LTU) refused to give up and set a new Olympic record for the Combined Event of 12:01.01.
Five riders enjoyed a maximum score of 300: McCann (CAN), Coyle (IRL), Kim Sunwoo (KOR), Kate French (GBR) and Tamara Vega (MEX). Leydi Laura Moya (CUB) suffered what looked like a nasty injury after falling across an obsctale, but she recovered and showed incredible courage to declare herself fit for the Combined Event.
Earlier, Nowacka (POL) won the Women's Fencing to take a 12-point overall lead into the Riding. She took the final bout of the Fencing Bonus Round to top up her impressive score from yesterday's Ranking Round. After two events she led the competition with 554 points ahead of second-placed Clouvel (FRA, 542) and third-placed Alice Sotero (ITA, 524).
History was made at the Deodoro Stadium as Bonus Round Fencing was introduced to the Olympic Games for the first time - on an impressive and futuristic stage.
Haydy Morsy (EGY) and Anastasiya Prokopenko (BLR) took part in the first bout and soon there was a perfect demonstration of how to take advantage of the Bonus Round by Samantha Murray (GBR), who won a remarkable eight matches to move from 31st place in Fencing to 25th.
Anna Maliszewska (POL) was the other big winner as she won four bouts to give herself four extra seconds in the Combined Event.
Gulnaz Gubaydullina (RUS) won the Swimming event and set a new Olympic record. She recorded a time of 2mins 07.94secs in the final heat to finish ahead of Clouvel (FRA, 2:08.62) and the current world champion Sarolta Kovacs (HUN, 2:09.02).
Kovacs (HUN) was the previous record holder - she timed 2:08.11 at London 2012. And Clouvel (FRA) remains the world record holder with 2:06.59.
Before the incredibly fast finale - which resulted in the second of three Olympic records already at these Games - there were impressive wins in earlier heats by Esposito (AUS) in 2:12.38 and Elena Potapenko (KAZ) in 2:11.52.
When Esposito (AUS) won that heat, on the back of a fine Fencing performance at the Youth Arena yesterday, it was clear that she was in a position to challenge.
She delivered on that promise in spectacular style, and as a result Modern Pentathlon has a new superstar.