Gillum, who was trying to become the state's first black governor, told a crowd gathered on the campus of Florida A&M University, "I sincerely regret I couldn't bring it home for you".
Florida Democrats picked up two congressional seats in South Florida and could flip a key Tampa state Senate seat, though that race is headed for a recount.
DeSantis' victory as the next governor of Florida will also have a lasting impact on the state's Supreme Court.
But in Wisconsin, Democrat Tony Evers pulled off a narrow win in unseating Republican incumbent Scott Walker, according to data provider DDHQ.
The losses tempered big Democratic victories in governor races in two states, MI and Kansas, that voted for Republican President Donald Trump in 2016, bolstering the party's hopes of recapturing those states in the 2020 presidential election.
Republican former U.S. Rep. Ron DeSantis and his wife Casey voted early Tuesday morning in Ponte Vedra Beach.
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"I want to continue to build my relationship with Money Team Tokyo", Mayweather said in a press conference in Japan on Monday. ESPN reporter Ariel Helwani said it was unlikely to be an MMP bout.
Gillum - the mayor of Tallahassee who has the charisma and political acumen that led many to believe he would be the one to finally retake the Florida governorship for the Democrats - came into Tuesday's election soaring high by most pollsters' calculations.
Republicans, meanwhile, were eyeing potential pickups in CT and OR, both traditionally Democratic states.
Florida is the most populous of the battleground states that swing between the parties in presidential elections, but Republicans have controlled the governor's mansion for two decades.
"We could not be more thankful for the support that was shown by each and every one of you all the way along this path".
"Other than serving in the military, becoming the 46th governor of the great state of Florida is the greatest professional honor of my life", DeSantis said. That changed in the final week, when Trump helped drive out Republican supporters.
DeSantis, 40, was considered an underdog before Trump tweeted his support for DeSantis in December, a month before he got into the race. DeSantis also secured the endorsement of the Florida Realtors following the August 28 primary. County-level results showed Mucarsel-Powell won enough ballots in Miami-Dade to overcome Curbelo's advantage in the Keys. Despite implications that DeSantis was racially insensitive - an idea he angrily disputed during a debate - he was able to win the state that Trump carried in 2016.