This places it before the so-called Clovis hunters, who many academics had argued were the North American continent's original inhabitants.
News of the dating results is reported in Science magazine.
In truth, the "Clovis first" model, which holds to the idea that America's original human population swept across a land-bridge from Siberia some 13,000 years ago, has looked untenable for some time.
A succession of archaeological finds right across the United States and northern Mexico have indicated there was human activity much earlier than this - perhaps as early as 15-16,000 years ago.
The mastodon rib, however, really leaves the once cherished model with nowhere to go.
The specimen has actually been known about for more than 30 years. It is plainly from an old male animal that had been attacked with some kind of weaponry.
It was found in the late 1970s at the Manis site near Sequim, north-west of Seattle, in Washington State.
Although scientists at the time correctly identified the specimen's antiquity, adherents to the Clovis-first model questioned the dating and interpretation of the site.
To try to settle any lingering uncertainty, Prof Michael Waters of Texas A&M University and colleagues called upon a range of up-to-date analytical tools and revisited the specimen.
These investigations included new radio carbon tests using atomic accelerators.