Ireland made 221 for nine, with Jeremy Bray scoring an unbeaten 115, before a dramatic finale saw them claim four wickets and restrict their opponents to just 12 runs off the final four overs.
Johnston said the entire day, with hundreds of Irish fans singing and cheering in Sabina Park had been hard to take in.
"The first hour was surreal -- just being at the World Cup, seeing the cameras and the fans going crazy -- it was just an unbelievable experience and it is just going to get better" the Australian-born Johnston told a news conference.
Ireland face Pakistan on Saturday and then hosts West Indies in their remaining Group D games in the Jamaican capital.
"We've come here to promote cricket in Ireland and we have shown there is good cricket in Ireland and given people an entertaining game to watch," he added.
Johnston was full of praise for his side, singling out the approach of the bowlers of the dramatic final two overs -- medium pacer Kevin O'Brien and off-spinner Andrew White.
"They are two gutsy cricketers," said Johnston, "There was no shake of the head when I threw them the ball and that is what this team is all about and why we can hang tough like that," he added.
"I thought 221 was enough but of course it depended on how we bowled but I knew if we could squeeze them we could put them under pressure.
"We got a bit of luck when (Brendan) Taylor was run out and that was the lifeline we needed," said the Irish skipper referring to the dismissal of Zimbabwe's wicketkeeper when the African appeared to be heading comfortably to victory.
"We squeezed them and then the bowling at the end was outstanding in the last eight overs," he added.
Ireland's performance was accompanied by plenty of singing and dancing from a high-spirited travelling support and even though the crowd barely passed 2,000 Johnston said they had made a difference.
"We knew they were coming and there will be more coming for the game on Saturday. They cheered every dot ball and bit of fielding. They were liked 12th and 13th men for us. That is what got us home in the last few overs," he said.
