Austen Harrison was crushed by the furniture at the Hugo Boss shop in Bicester, Oxfordshire, in June 2013.
Oxford Coroner's Court heard the large mirror was not fixed to the wall and toppled over as Austen played with it.
He had emergency surgery to relieve pressure on his brain, but his life-support was turned off four days later.
Austen, from Turners Hill in Crawley, West Sussex, was with his parents, Simon and Irina Harrison, on 4 June.
In evidence read to an inquest jury by coroner Darren Salter, Mr Harrison said he had been trying on a number of suits in the shop.
The court heard Austen had been playing with the free-standing mirror, which had winged sections on either side attached by hinges.
Mr Harrison said he was not concerned for his son's safety because he "couldn't think of any reason why such a large mirror wouldn't be fixed to the wall".
He said he and his wife heard an extremely loud bang, "as if something had fallen over", and someone gasped.
"I instantly knew Austen was lying underneath it, as it was not lying flat on the floor," he said.
Mr Harrison lifted the mirror from his son, who was lying motionless.
He said Austen had been lying "face-up with his legs straight out and his arms by his side".
Austen was taken to the John Radcliffe Hospital in Oxford and underwent emergency surgery for swelling to his brain.
Doctors told his family Austen would never recover from the "irreversible" brain damage.
His life-support was switched off and he died at 05:45 BST on 8 June.
The inquest continues.
A four-year-old boy died from "devastating" head injuries after an unstable shop mirror fell on top of him, an inquest has heard.