Mr Key said he was a rag trade veteran, having worked for clothing manufacturer Lane Walker Rudkin for two years.
Kilt continues to grow its New Zealand Made business, with 10 retail outlets nationwide.
Mr Key took off his New Zealand-made suit to sew a hem using an industrial sewing machine.
Production manager Cushla Samson admonished him for not knowing the Wellington boutique's location.
"I'm sort of a caged animal - they don't let me out," he said.
Mr Key promoted a design change to New Zealand's flag, to include the Silver Fern, to both Kilt staff and secondary school students later in the day.