Shoppers responded to the deals and bargains they found online, comScore Chairman Gian Fulgoni said. Whether they continue to do so in December isn't yet clear.
"While we anticipate that there will be more billion-dollar spending days ahead as we get deeper into the season, only time will tell if overall consumer online spending remains at the elevated levels we've seen thus far," he said in a statement.
Meanwhile, another company that tracks online spending, IBM's Coremetrics, found Cyber Monday sales rose 19.4 percent over last year.
Cyber Monday was also PayPal's biggest day ever. Online payments rose 19 percent from last year.
Though it is growing quickly, online spending makes up only 8 to 10 percent of total holiday spending.
The Cyber Monday figures come a day after a report showed American's confidence in the economy rose to a five-month high in November and is welcome news for retailers hoping that Americans start spending more freely. But shoppers are still holding out for bargains and spending cautiously as unemployment remains high.
According to ShopperTrak figures, revenue at stores in shopping malls was flat over the weekend following Thanksgiving, but traffic rose 2.8 percent.
ComScore said the number of buyers online increased at a slower rate than total spending, up 4 percent to 9 million. The average shopper spent 12 percent more, at $114.24 each, according to the data.
Cyber Monday got its name from the National Retail Federation trade group in 2005 to describe the unofficial kickoff to the online shopping season. The idea was that people returning to work after the long weekend would shop at their desks.
It never really was the busiest online shopping day of the year, but it has gained significance as sellers have trained shoppers to expect deals that day. Nearly 90 percent of U.S. retailers offered some kind of Cyber Monday promotion this year, compared to 72 percent in 2007.
While people used to shop at work to take advantage of broadband connections, the data shows even though broadband has become common at home, about 49 percent of the dollars spent at U.S. Web sites originated from work computers, down 4 percent from last year, comScore says.
Buying at home rose just 4 percent to make up about 45 percent of dollars spent. International shoppers on U.S. sites made up the rest.
The fact that buying at work remains so prevalent suggests they are doing it "to shop for holiday gifts while minimizing the risk that their children, spouses and significant others might see" them shop, Fulgoni said.
A clearer picture of spending will come Thursday when the nation's retailers release revenue figures for November.