Their habitat was perched between two of the world's largest volcanoes in Hawaii at 2700 metres above sea level. Each scientist had a bedroom about the size of an "under-the-stairs cupboard" and along with just 80 square metres of usable space - including a kitchen, dining room, two bathrooms, lab, exercise area and common areas.
As you could imagine, living in such close quarters led to some testy moments between the scientists.
"As every family knows you don't always get along all the time. We had plenty of arguments during the year," she said. "Working though that just to figure out how to get along just to get to the next stage was really important."
One of the drawbacks of being chosen to lead humanity's push to become an interplanetary species is having to give up your privacy.
"You could hear everything else that happened in the dome, and they could hear you," Johnston recounted. "It wasn't very private at all."
Living on top of each other made for some intimate, and rather icky, situations.
"It seems like every simulation or mission has issues with their toilet," she said. "It's always one of the big issues. So we had to clean the toilets every week ... we had to remove compost from there. It became a stinky issue rather than an unclean one."
About two or three times a week the team would leave the dome for the outside world, but of course that involved donning a heavy space suit to contend with the Martian conditions.
The make believe astronauts would receive a food drop about every four months with freeze dried, dehydrated and powdered ingredients from which they made their food.
"Ideally if you were on a trip to Mars you would have all sorts of life support systems and plant growth systems that could produce all of your food but (this) experiment was ... more geared towards the social and psychological aspects," she said.
The only contact they had with the outside world - and their loved ones - was via e-mail with a 20-minute delay each way to replicate the time it would take to get a message to Mars.
But of all things that surprised her about the what she missed it was avocados, for which she had a daily craving.
However, it might please you to know that the toughest thing for Johnston and other crew members was something we all experience as Earth-dwelling beings: getting stuff done. The days in isolation were strange to get accustomed to and the participants had to adjust to their new lifestyle and feel like they were achieving what they wanted to.
"If you have a bunch of over-achievers in one area, it's very easy for them to feel that they're not getting enough done," she said.
The six scientists filled out more than 3,000 surveys throughout the year, and while NASA has not yet released the findings from its intensive study, Ms Johnston will be in Australia in March.
She will speak at a number of events celebrating women, including the Sydney Opera House's All About Women festival in Sydney on 5 March, the Wheeler Centre in Melbourne on 6 March, and WOMADelaide's Planet Talks in Adelaide on 11 March.
