Victory in the tournament, which this year serves as a crucial shake down for six Pacific rim countries ahead of the World Cup, put Japan ahead of Fiji in the World rankings and into the top 10.
It was their third Pacific crown after winning in 2011 and sharing the title with Samoa in 2014, and the victory came after a three-year absence from the tournament.
Japan's set piece steadily improved through the competition and in the final they added a variety of moves in general play to counter the size dominance of the larger Americans.
"It all came down to preparation. We've been together since February and all the hard work has paid off," Leitch said.
"I think our discipline maybe let us down, but I think we're on the right track."
The USA finished strongly and coach Gary Gold said his team's World Cup preparation was on track despite the scoreline.
"We got tested by a such a good Japanese team. When you make mistakes they make you pay, and that was a good lesson for us," Gold said.
"But I'm really proud of our guys for how they fought right until the end, and I think we're in a good place."
Japan's first try, to Leitch, came after an 11-man lineout drive and they followed by cutting through the US defence with a series of short snap passes, until a gap appeared for Kenki Fukuoka to race through and score.
Fly-half Yu Tamura was on target with his kicking, landing both conversions and a penalty to have Japan 17-0 ahead after 11 minutes.
They then faced a strong fightback which saw a try from sevens star Madison Hughes and a conversion by Will Hooley, who also added a penalty while deputising for injured regular kicker AJ McGinty.
Tamura and Hooley traded penalties in the final minutes of the half for Japan to turn 20-13 ahead.
Japan made another explosive start to the second half with an inside pass from Tamura sending Ryohei Yamanaka over.
Fukuoka then latched on to a wayward American kick on the Japan 22 to spark a counter-attack that swept 80 metres, with Leitch on hand at the finish to score his second try.
The introduction of replacement forwards worked to the USA's advantage, with a superior scrum resulting in a late try to Hanco Germishuys.
Fiji, who had won the tournament for the past four years, were beaten by Japan in pool play and had to be content with winning the play-off for third against Samoa 10-3.
In a bruising encounter, Harlequins wing Vereniki Goneva scored the only try in the match early in the second half.
Samoa had the better of territory and possession but Fiji coach John McKee was pleased to see they could not crack his side's close marking defence.
"We won a very physical arm wrestle. In our attack we would have like to develop more continuity," McKee said.
"But we were under pressure defensively for parts of the game and credit to our players I thought our defensive system worked well."