The death toll is likely to rise as another 109 people remain missing, Kentucky Governor Andy Beshear said.
Some 28,000 Kentucky homes and businesses still lacked power on Monday evening local time.
Friday's tornadoes, which the governor estimated destroyed 1000 homes, surprised people by striking unusually late in the year.
"It may be weeks before we have counts on both deaths and levels of destruction," Beshear told reporters, adding the victims ranged in age from five months to 86 years old.
It has proven difficult for authorities to pin down the exact death toll.
Piles of wreckage, interruptions to mobile phone service and the number of people sheltering with friends and relatives have complicated efforts to identify bodies.
While Kentucky bore the brunt of the tornadoes, including one that tore across 365km of terrain, six people died in Illinois , four in Tennessee and two in Missouri, while a nursing home was struck in Arkansas, causing one of that state's two deaths.
Across Kentucky, neighbours and volunteers worked to house, feed and offer any other assistance to those left homeless by the storm.
Homes across the town had collapsed walls, missing roofs and uprooted trees scattered across lawns.
The police and fire stations, as well as a local candle factory, were obliterated.
President Joe Biden will travel to the state on Wednesday to visit hard-hit areas including Mayfield, the White House said.
While the National Weather Service has yet to conclude the strength of the twisters that tore through Mayfield, Governor Beshear said they were probably so powerful that no amount of training or advanced notice would have made a difference.
"You can have the warnings, but what do you do?" he said.
"How do you tell people that there's going to be one of the most powerful tornadoes in history and it's going to come directly through your building?".
In a flicker of good news, Beshear said the death toll from Mayfield's collapsed candle factory may be lower than officials first thought.
He said authorities were trying to confirm information from the owners of the Mayfield Consumer Products site that eight people died when the storm hit, with only a small number of the 110 workers unaccounted for.
"We feared much, much worse," he said. "I pray that it is accurate."
Kentucky's emergency management director Michael Dossett said 28,000 homes and businesses remained without power.
More than 300 National Guard personnel and scores of state workers were distributing supplies and working to clear roads so mountains of debris can be removed, the governor said.
Beshear said the search, rescue and recovery process had been an emotional rollercoaster for all involved.
"You go from grief to shock to being resolute for a span of 10 minutes, and then you go back," he said.
While Kentucky was hardest hit, six workers were killed at an Amazon warehouse in Illinois after the plant buckled under the force of the tornado.
