Darryn Frost took the tusk from the wall of the City of London venue and used it as a makeshift weapon when Khan launched his killing spree on November 29, 2019.
He and others tackled Khan inside the hall, then pinned down the “predator” on London Bridge to stop him from attacking members of the public.
Ultimately, Khan was shot dead by armed police who feared the fake suicide bomb device he had strapped to his chest was real.
Giving evidence today, Mr Frost, a communications manager for the Prison and Probation Service, described the moment he came face-to-face with Khan after hearing the screams of stabbing victims.
“The first thing I saw was these two shining blades – they were above his (Khan’s) ear level – he was holding them up and kind of bashing at the door.
“I didn’t attack, I put the tip of the narwhal tusk about an inch or two above his midriff.”
As the incident unfolded, Mr Frost realised Khan was wearing a device around his torso that he claimed was a bomb.
When armed police arrived, he had Khan pinned down by his arms, and told the inquest he wanted to stop the extremist from getting his “expected outcome”.
“I said: ‘I’ve got his hands, he can’t kill anyone else, I won’t let him kill anyone else’. I didn’t want him to be shot. His statement that he was waiting for the police meant he wanted to die.”
Struggling to contain his emotions, he added: “I saw the chaos he had caused in the hall – I didn’t want him to have the satisfaction of his choice when he had taken that away from others.”
The terror attack unfolded during an anniversary celebration of the Learning Together programme, a Cambridge University initiative to help prisoners through education.
Jack Merritt, 25, a Learning Together coordinator, and 23-year-old Saskia Jones, who had also worked for the programme, were both stabbed to death by Khan.
Mr Frost recalled the “call to action” after hearing the initial screams, and said he scoured the historic hall for an item to use as a weapon.
“My immediate thought was a chair, but thought that was not enough”, he said.
“I glanced over to my left… on either side of the door were these really long narwhal tusks – I thought they were whale tusks.
“There were these two long, white, shining objects. I took the left narwhal tusk out of the bracket.”
He joined conference delegate Steve Gallant who was already trying to subdue Khan with a mahogany chair, and then was involved in the pursuit on to London Bridge.
Mr Frost likened Khan’s pursuit of members of the public on the bridge to “a shoal of fish being chased by a predator”, and said another delegate John Crilly used a fire extinguisher in an attempt to compromise Khan’s vision.
“He raised the knives up again and started running towards me”, he said.
“He was trying to attack me and I thought I would need to keep some distance.
“As he raised his arms there was a tiny piece of skin that showed on his left. Time slowed down so slowly and I was able to aim for that centimetre of flesh.
“I aimed the tip of the tusk at him, and as he was running towards me I thrust at him.”
He said Steve Gallant, covered by the spray from the fire extinguisher, was able to wrestle Khan to the floor despite being unarmed.
Describing the confrontation, Mr Crilly said he narrowly avoided being stabbed in the hand as Mr Frost knocked Khan off balance with the tusk.
When police arrived, he said he told officers to “shoot the bastard”.
The inquest continues.