Tunku Ismail Idris said he witnessed the attack, in which two people died, from a nearby hotel lobby.
“People were shouting and running into the hotel from the mall when the shooter started firing,” Ismail wrote on Facebook just hours after the shooting spree had happened.
He said he and his security team had carried his family to safety in the hotel basement.
“The only thing on my mind was how many shooters were there, what weapons they had and what was the situation,” he wrote. “We had to be on alert and be prepared for any circumstance because all we knew at that point were the gunshots we heard.”
Thai police on Tuesday arrested a 14-year-old boy suspected of killing two people – a Chinese tourist and a Myanmar national – and wounding five others at the Siam Paragon shopping centre.
The major shopping and entertainment venue popular with tourists and locals houses high-end stores, a cinema, an aquarium and a five-star hotel.
Ismail said images of him telling his children “everything is going to be OK” as they waited for their car still haunted him.
“My security team and I stood in front of my family, creating a human shield to protect them at all cost,” he said.
Ismail, who leads the Royal Johor Military Force – a tiny private army that serves as the sultanate’s royal guard – also commended his wife for consoling their horror-stricken children.
“I was shouting at the top of my voice to our security to get the car to the basement and get us to a secure location away from the place,” he said.
The security team took the members of the royal family to Singapore’s Bangkok embassy after the driver told them it was closer than the Malaysian mission.
“I called the Malaysian prime minister and our defence minister to inform them what was happening. The Malaysian ambassador is also with us,” he said.
“Sadly, I missed the JDT game but I’m glad my family and our team are safe. This is the worst experience I’ve ever gone through: protecting the lives of my children from a killer.”
Early on Wednesday, he said in a post on X (formerly Twitter) that his family had landed safely back in Johor.