Martin McElroy, a Labour councillor in Glasgow, locked himself in a back room in the cigar shop with several other customers, forcing the door shut with a table.
Mr McElroy said the gunman had been just 5ft from him when the other man was shot. The gunman then left the shop but returned, firing at his victim again, sparking panic in the specialist store in north Houston.
The injured man, who had been shot in the torso and hands, had to be flown to hospital, with local reports in Harris County saying his condition was critical.
Mr McElroy, who represents the Hillhead ward in the west of the city and was on the last night of a holiday, also told of his fear when the police failed to arrive on the scene.
He said: "We didn't know if the guy was on a rampage or not, so it was pretty frightening.
"We tried all the doors at the back to get to the emergency exit but they were all locked and about seven of us had to hide in room with a poker table pushed up against the door."
Mr McElroy had been visiting childhood friend and oil-industry worker Liam Boyce, 26, from Fife, in Houston for a week and had been in the cigar shop on Tuesday night buying souvenirs when the incident occurred. He said a fight had broken out which "looked a bit like handbags" but forced the shop worker to phone 911.
One of the men involved in the fight had prevented the other leaving by blocking the door until the police arrived.
Mr McElroy said: "Next thing we hear is the big guy guarding the door getting shot in the chest. The shooter then left the shop.
"When I looked over at the guy who was shot the blood was starting to seep out his shirt and just as I was about to jump up and phone the paramedics the guy came back and shot him again.
"This time a whole bunch of customers who were sitting on sofas within the shop all started running into the back of the shop, and myself and Liam made a break for it too."
Authorities in Texas said they do not know the motive but said one man involved was the ex-husband of the other's current girlfriend.
Mr McElroy added: "We were pretty shaken up but unhurt. I phoned my mum from the police car.
"Liam and I have been friends since high school so my mum knows the scrapes we get into. But this took it to a new level."