Mohsni connected with a pin-point Stevie Smith free-kick with just a few minutes remaining in the showpiece match at Easter Road on Sunday.
But the centre-half somehow sent his effort over the crossbar from a few yards out - with an empty net beckoning invitingly.
He said: "My heart was in my mouth when Mohsni had his header late on.
"It was a great chance and I thought he had put it in the back of the net.
"They had a few decent chances to score, but couldn't take any of them. I thought it was going to go to penalties again and I wasn't looking forward to it.
"It was obviously just destined to be our day. To beat a club like Rangers in a cup final is just absolutely incredible. We defended brilliantly."
Robinson, who started his professional career as a kid at Rangers, won the cup with Queen of the South after a penalty shoot-out last season.
He feels the time he spent at Ibrox as a youngster has been invaluable for him even though he did not manage to break into the first team.
The shot-stopper said: "They were some of the best years of my life. I learned off some of the best players in the game and improved and developed as a player enormously.
"I was disappointed not to hold down a place in the first team, but I have definitely benefited from my time there."