The Nashville-born player crossed in the corner in each half following passes from Ruaridh Jackson as Scotland survived a fright from their opponents early in the second half to open their viagogo Autumn Test series with a win.
Among the Murrayfield crowd was Seymour's Northern Ireland-based parents, including Glasgow-born mother Sue, and the 25-year-old was among the proudest men in the country after marking his third cap in memorable fashion.
Describing the feeling when he touched down in the 32nd minute, Seymour said: "Amazing. A really, really special feeling.
"I was just delighted to cross over and my parents, brother and girlfriend were in the crowd so it was perfect. My parents hadn't seen me play for a while so it was a special moment that they were there to witness that.
"You are proud but hopefully I have done them proud as well. I'm very happy. It was a wonderful experience and hopefully I get to repeat it at some point."
Seymour joined the Scotland squad for the first time in the summer, playing twice in their tournament in South Africa.
"It's been great, very fast-paced and very exciting," he said. "You have got to take in the whole occasion. It's just nice going out there with that badge on my chest and making sure you are thinking about all the people I'm representing past and present.
"It's a great feeling and it's something I will never get sick of doing."
Despite having two caps under his belt, Seymour admitted he felt nervous ahead of his Murrayfield debut.
"I felt the nerves coming off the bus and going out there in front of the crowd because you want to do well," he said.
"Your home debut, you want to go out there and make Scotland proud and fill a jersey the way the country is talking about. But you can't dwell on that, once the whistle goes you are pretty much into it."
The game was not without its problems for Scott Johnson's side. They struggled to get into a rhythm in the first half on a pitch which cut up badly amid an ongoing worm infestation, although Seymour's first try helped them to a deserved 11-3 half-time lead.
But Japan twice cut the deficit to one point when Kenki Fukuoka crossed as the visitors finished off sharp moves.
Each time Scotland hit back almost immediately, through Greig Laidlaw and Seymour, and the hosts broke clear in the final 20 minutes when Alasdair Dickinson, Duncan Weir and Sean Lamont, who had a first-half try disallowed, all crossed when Japan were down to 14 men following separate yellow cards.
Johnson, whose successor Vern Cotter was watching in the stand, was torn between a positive and negative frame of mind after watching his team hit back quickly when defensive frailties were exposed.
But Seymour said: "As a whole we are very happy - 42 points is a great number of tries to come away with. That's obviously positive for us.
"There are a few things we need to make better and get more consistent but personally I'm very happy and the performance from the boys as a whole was very good."
Scotland meet South Africa at Murrayfield next Sunday but Matt Scott could be struggling to face the Springboks after sustaining a hand injury in the closing stages.
The Edinburgh centre is to travel to Leeds to visit a consultant hand surgeon.
Meanwhile, Glasgow forwards Alastair Kellock and Ryan Grant, who went off in the first half after taking a knee to the head, both suffered concussion.
Man-of-the-match Tim Swinson (shoulder) and Weir (groin) will also require further assessment.
- Sports & Recreation