Two couples were reportedly watching Lone Survivor at The Grove 16 cinema in Wesley Chapel, north of Tampa, when 71-year-old Curtis Reeves asked Chad Oulson, 43, to stop texting. Police said the argument that followed led to the shooting and Mr Reeves has been charged with second-degree murder.
Mr Oulson and his wife Nichole, who was also shot and injured, were taken to hospital where Mr Oulson later died.
An off-duty police officer detained Reeves until colleagues arrived.
Charles Cummings, who was also at the cinema, with his adult son, said a man in the back row got up and left the auditorium, presumably to get a manager.
But he came back alone after a few minutes, appearing upset, and the argument between the two men resumed.
A man sitting next to the gunman grabbed the weapon out of his hand and the suspect did not try to flee.