Steyer also will attend Gillum's victory party on Tuesday, Mack added.
Gillum's campaign did not immediately respond to CNBC's request for comment.
The move comes after Steyer invested $14 million into the race in support of Gillum and as polls begin to show that Democrats may have a chance at flipping the Florida governor's seat. Steyer and his wife Kathryn have combined to give $50 million in support of Democratic causes this cycle, according to the Center for Responsive Politics.
The latest Quinnipiac University poll shows Gillum up by seven points over his Republican opponent, Congressman Ron DeSantis.
Previously held by Republican Senate candidate Rick Scott, GOP strategists went into the race thinking they would easily hold on.
However Gillum, the mayor of Tallahassee, has found a way to surge ahead by appealing to voters with a Medicare-for-all pitch and bettering the state's education system.
He's also had help from other Democratic leaders including former president Barack Obama, who held a rally for Gillum and Florida Democratic Senator Bill Nelson on Friday.