The Miami Herald reported Ferguson's death at the state prison in Starke, officially clocked at 6:17 p.m. EDT, came about an hour after the nation's highest court declined to intercede. His final words could not be understood by witnesses, the newspaper said.
He was the fifth Florida death row inmate to be executed since December.
Earlier in the day, Ferguson met with two of his attorneys and a spiritual adviser, a corrections spokeswoman said.
Ferguson's last meal was a meat patty with white bread, steamed tomatoes, potato salad, diced carrots and iced tea, the Herald said.
Christopher Handman, Ferguson's attorney, said the defense team was "gravely disappointed" the Supreme Court denied a stay of execution on the grounds of mental illness.
"Mr. Ferguson has a documented 40-year history of severe mental illness diagnosed repeatedly by state doctors in state institutions," Handman wrote in a statement. "Mr. Ferguson has been profoundly mentally ill for four decades, pre-dating the crimes for which he is scheduled to be executed, but is now deemed suddenly and inexplicably cured."
Ferguson, 65, was one of a group of armed men who killed six people in a Carol City, Fla., home-invasion robbery in 1977, and he killed two Hialeah, Fla., high school students in another robbery in 1978.
Ferguson received a stay of execution 10 months ago so the courts could examine whether his death would constitute "cruel and unusual punishment." His lawyers said Ferguson, who believed he was the "Prince of God" and had been diagnosed as paranoid schizophrenic, was unaware of why he was being put to death.
The American Bar Association and the national alliance on Mental Illness each filed briefs asking the high court to halt the execution, the Herald said.
The U.S. 11th Circuit Court of Appeals rejected his appeal in May, ruling Ferguson was competent to be executed.