The lock had been named in Eddie Jones' squad after helping Saints in their push for Premiership and Champions Cup glory.
The black, green and gold were chasing a top-two finish in the league and had just bagged a brilliant win in Lyon to set up a European quarter-final at Exeter Chiefs.
But fast forward 12 months and Moon is left to look back on a turbulent time.
His club fell down the Premiership standings in a Covid-19-hit 2020 and lost to eventual champions Exeter in that Champions Cup last-eight clash.
Moon suffered a knee injury against Bristol Bears last February that stopped his momentum and he admits he struggled to regain his best form after rugby emerged from isolation in August.
But the 24-year-old remained on England's radar and made it into their autumn squad while helping Saints to start to rebuild after a tough run.
Moon may not have received a Six Nations call this year, but he certainly seems to be a man on the up again.
And he took time to look back on the past 12 months ahead of Saturday's Premiership trip to Gloucester.
"I had a pretty awesome start to 2020, we beat Lyon away, were top four at the time and we qualified for the quarters of Europe," Moon said.
"Literally a couple of days after that game in Lyon I got the notification I was in the Six Nations squad and then went to Portugal (for the training camp).
"I came back, did my knee injury then didn't play from then until August and I just didn't play my best rugby during that Covid block when we were playing three games a week.
"I was fortunate enough to get back into camp and I tried to find my form through training, played well against Leinster and it was a weird year for me really.
"I started off on a high, then had a big low, like basically everyone in the country, and then it ended on a high.
"I look back at it and it's been weird but I can't complain.
"Being called up to the England squad a couple of times is not the worst thing in the world!"
So does Moon feel like he's starting to get back to where he wants to be now?
"Yeah, for sure," he said.
"Even when we were playing well, a team would get off to a quick start against us and we'd almost lose it at that point and the big thing for us is coming out and scoring first after half-time.
"Against Leinster, they scored straight after half-time and a year or two ago that would have made us crumble but we've started to show a lot of head strength.
"It's been a big thing for us and then getting the win against Worcester the week after was huge."
Moon is part of a Saints second row department that is extremely strong.
He and David Ribbans are the fringes of the England setup, Courtney Lawes is in it, Api Ratuniyarawa is a Fiji international, Nick Isiekwe is a star performer, Alex Coles is a player with huge potential and Lewis Bean has done well on loan at Glasgow Warriors of late.
And Moon said: "At other clubs, there might be second row rooms where players don't want to help each other because they may give others the edge, but it's not like that at all here.
"We've got a group chat called lineout nauses where we're all constantly talking with each other about lineout stuff or stuff around the field and (Saints forwards coach) Dows (Phil Dowson) is really good at leading all that, being the admin.
"Dows has got to manage expectations because if four or five of us are playing well we'll expect to be in the team.
"He tells us where we stand and why we're in the team or not and we're in a good situation with that at the moment."
Saints head to Gloucester on Saturday and Moon knows what to expect at Kingsholm.
"They're a massive maul threat," he said.
"As soon as they get that going they can run all their plays off of it.
"For us forwards wise, it's very much on us.
"We're going to be rusty because it's five weeks since we played but mauling can help us overcome that.
"You can always be physical and maul and if we can nullify theirs and get ours going, it will be good.
"We had a training game here last Friday and it was a big emphasis for us and we know that mauling game is their biggest threat."