Four people were killed at the scene, while a fifth died at the hospital, Orange County Sheriff Jerry Demings told reporters. Seven people survived the ordeal.
The business, Fiamma, makes awnings and other accessories for recreational vehicles.
Demings said the incident was being treated as a "workplace violence incident" and did not appear to have any links to terror.
The 45-year-old shooter, identified by police as Army veteran John Robert Neumann Jr, had a semiautomatic handgun and two knives in his possession. All of the dead were former co-workers.
Shelley Adams, a resident of the area, told local media her sister worked at the firm and had been in the bathroom when she heard shooting.
"She came out and saw a man on the floor," Adams said.
The woman then called her sister on the telephone and kept saying, "My boss is dead. My boss is dead," Adams said.
The assailant had allegedly beaten up another employee at the business in 2014, but no charges were filed, the sheriff told reporters.
"It is a sad day for us here once again in Orange County," Demings said.
A year ago, Orlando – a hub of Florida’s resort industry popular with tourists – was the scene of a mass shooting in a gay nightclub that left 49 people dead and dozens more wounded.
The gunman in that attack, Omar Mateen, had declared allegiance to the Islamic State group shortly before going on his killing spree.
"I ask all Floridians to pray for the families impacted by this senseless act of violence," Florida Governor Rick Scott said in a statement.
Neumann, who had no known family and was living alone at the time of the attack, was an ex-soldier who had been honorably discharged from the US Army in 1999.
The reason for his dismissal from Fiamma, where he had worked for years, was not immediately known.
Police said he had walked into the offices through a back door as his former colleagues were arriving for work at around 8:00am (1200 GMT) on Monday and opened fire, reloading his gun at least once.