The Olympic gold medalist beat World Cup slalom champion Mikaela Shiffrin by 0.88 seconds in the opening event in the four-day competition at the 1960 Olympic mountain.
"It's very fun to be at my home mountain and, of course, I want to win here," Mancuso said. "I think that's the cool thing about records even if it's something like U.S. Championships. When you are kind of pushing for something or extending a record, It makes it that much more fun.
"I've seen a lot of people I know and all my friends come out to cheer me on. And all the kids, it's really, really fun. It's even a school day, so they're probably happy they got to miss school."
The 18-year-old Shiffrin, from Vail, Colo., won the World Cup slalom title last week in Switzerland.
"It's awesome to be here," Shiffrin said. "It's beautiful today and hopefully this weather keeps up. I love U.S. Championships, it's such a fun event and there's so much healthy competition."
Megan McJames, from Park City, Utah, was third in the race that was shortened from the planned start because of inconsistent snow after heavy snowfall Wednesday.
"This year's been tough," McJames said. "I'm not on the U.S. ski team, so we've been figuring it out as we go along and it's been pretty rewarding, pretty fun. Just a new challenge, but it's exciting. I love ski racing, so that's why I'm still out here.
Tim Jitloff won the men's giant slalom for his fourth U.S. title. He grew up in nearby Truckee and now lives in Reno, Nev.
"It was challenging for sure, but I grew up racing on this hill, so I know what's going on," Jitloff said. For me, this is just a lot of fun, so it's really a weight off the shoulder if you can go and just enjoy yourself. That's what it was today. It felt good to kind of reel back the years a little bit on home turf."
Defending champ Robby Kelley of Starksboro, Vt., was second.
"I definitely wanted to repeat after winning last year, but it's definitely a good accomplishment, coming here and getting second," Kelley said. "Jit has been skiing great all year, so I kind of expected it."
Seppi Stiegler, from Jackson Hole, Wyo., was third.
Ted Ligety, the World Cup and world champion, didn't race.
Laurenne Ross won super G on Friday in the U.S. Alpine Championships, beating Stacey Cook by 0.01 seconds for her first national title.
Defending champion Julia Mancuso was third, 0.08 seconds behind Ross.
"Everybody always goes out there with the goal of getting on the top of the podium, but usually it's Julia," said Ross, from Bend, Ore. "It's nice to beat her every once in a while because it doesn't happen very often and when it does, it's really exciting.
"This was the toughest U.S. Championships super G course I've ever ran. There are a couple really technical sections. It never lets up. You always have to be on your toes and look ahead."
Cook is from Mammoth Mountain.
"It wasn't a feel-good run at all, but it's the time that matters in the finish," Cook said. "It's not necessarily how you look on the way down, it's how fast you can get here."
Mancuso, from Squaw Valley, won the giant slalom Thursday.
"There's some difficult turns on top and it's still bumpy from the new snow we got, but it was still a lot of fun," Mancuso said. "I wanted to make it to the finish and win, but our World Cup team is super tough, so it isn't a surprise that Laurenne and Stacey were faster."
The race served as the first half of the combined event, pairing the fastest times from super G and slalom.
In the men's race, Squaw Valley star Travis Ganong won by 0.20 seconds on his home course. Ganong also won the event in 2010.
"It's really nice to actually be home and on home snow, sleep in my own bed, and have all my family and the hometown crowd out supporting," Ganong said. "I grew up training on this hill every day, so it's really nice to get on a familiar hill. The hometown crowd really motivates me to push it harder and really go for it.
"This is more of a World Cup than a nationals hill. It's pretty tricky, really steep out of the top with some blind turns. It's intimidating. The key sections for me were on the flats."
NorAm overall and U.S. downhill champion Jared Goldberg of Holladay, Utah, was second, and Mark Engel of Truckee finished third.
"The course is really good," Goldberg said. "This is a lot like what I'm used to. There's blind rollers everywhere. It's like everything that we've always trained and I was super excited to ski it. It's one of the more fun courses I've run."