The USWNT veteran is retiring at the end of the season and, with the Reign sitting in the final playoff position as the NWSL wrapped up its regular season, there was a chance that the game in Chicago could be her last as a professional if results hadn't gone their way.
However, two Rapinoe goals early in the second half and a third from teammate Jessica Fishlock soon afterward moved the Reign into fourth place in the standings and have them heading to the postseason.
"It just means the world," Morgan said. "We came into this game just acting like if we do our job and get a little luck then we've got this. I feel like we dominated from start to finish. It was really a complete game for this team and I'm really proud of this team."
The Wave's move into first came at the expense of the defending champion Portland Thorns, who lost 5-1 to Angel City FC -- a result that moved the Los Angeles club into a playoff spot of its own with a fifth-place finish.
Despite the loss for the Thorns, last season's MVP and Portland's star striker Sophia Smith won the Golden Boot for most goals with 11.
Angel City has lost just once in 11 league matches since interim head coach Becki Tweed took over June 14, and it got the job done against Portland in front of sellout crowd of 22,000 at BMO Stadium with Sydney Leroux getting onto the scoresheet.
"It's been a tough year, and I think this is what everyone has been waiting for, a dominating performance by Angel City," said Leroux, who was sidelined with injuries for much of the first part of the season. "I'm just so happy that I got to be a part of that and do it at home. Now we're in the playoffs and there's tears and all of the things.
"Today was amazing. You have some tough times and then you have a day like today and you're like, 'OK, it's all worth it.'"
Gotham's Ali Krieger played 90 minutes in her final regular-season match before retiring at the end of the season, becoming among just four players to log 15,000 NWSL minutes. She was honored in a postmatch ceremony.
"Honestly, it's been so overwhelming in the most positive way," the two-time Women's World Cup winner with the U.S. said. "All of the love and the support has been -- I'm honestly speechless because you don't realize how much you've impacted all of these people over the years that I've played. It's so rewarding to see all the positive feedback and the love I've received just from playing the sport that I love."
The winners of the two quarterfinal games will go on to face either the Wave or Thorns -- who earned opening-round byes for finishing first and second, respectively -- in the semifinal round Sunday, Nov. 5.
Information from The Associated Press was used in this story.