The holders were without Megan Rapinoe for the semi-final in Lyon, the scorer of all her team's goals in the wins over Spain and France in the previous two rounds missing out with a hamstring injury.
That meant Christen Press came in and duly scored the opener, and it meant Morgan wore the captain's armband on her 30th birthday as she headed home Lindsey Horan's cross in the 31st minute for the deciding goal in a 2-1 victory watched by more than 53,000.
It was Morgan's sixth goal of the World Cup, putting her level with Ellen White -- who had earlier scored England's equaliser -- at the top of the scorers' chart.
However, it was a much-needed goal for the striker, who had not found the net since scoring five in the record-breaking 13-0 humiliation of Thailand with which the USA opened their defence of the trophy.
"I wouldn't say it was important but for me obviously it felt good to get on the scoresheet and to get the game-winning goal. We had the goals early and we had to hold on," said Morgan, who is back in Lyon having had a spell with Europe's leading club side in 2017.
Morgan celebrated her goal by miming sipping a cup of tea, which some saw as an attempt at a wind-up towards the English.
"'Pino' has so many celebrations and nobody asks her about them. I have only one and everyone wants to know," Morgan said, referring to her fellow co-captain Rapinoe.
"Obviously we have fun with celebrations and we had to keep it interesting."
'Saved our asses'
That goal was not the end of it, however, as the USA were grateful to the VAR for disallowing another White goal midway through the second half and to goalkeeper Alyssa Naeher for saving Steph Houghton's 84th-minute penalty, awarded after another VAR review.
"She saved our asses in that moment and we knew after that moment that the momentum was shifting and that we had to put in a huge amount of defensive work to close out the game," said Morgan of Naeher.
Press also earned praise for having a "tremendous game", and Morgan can now look forward to appearing in a third straight World Cup final.
"Yeah, it's pretty incredible. Each tournament for me has given me something different and I've looked at it through a different lens," she said.
Morgan was 22 when she came off the bench to score in the 2011 final against Japan in Frankfurt, which her team ended up losing on penalties.
In 2015 she was in the starting line-up for the 5-2 win over the Japanese in which skipper Carli Lloyd netted a hat-trick.
"Obviously I have a greater role being a captain on this team and leading this team in a different way so for me it's just so great," she added.
"I just saw all my teammates being so emotional after the game. We have put so much into this journey together and now we have one more game to close it out."