After coming second at the European Championships in Düsseldorf in June, she went on to compete against the best triathletes from across the world.
Mrs Derrick, the owner of Mary Derrick Swimming, based at Westonbirt Leisure Centre,came second in the 60-64 age category for the female sprint distance, and this is only her first year competing for Team GB.
The female distance sprint triathlon is made up of a 750-metre swim, a 20-kilometre bike ride, and a five-kilometre run.
She completed the swimming in 15 minutes 50 seconds, cycling in 41 minutes 16 seconds, and running in 22 minutes, the fastest in her age group.
Mrs Derrick was just beaten by fellow team mate Ann Hughes.
“I’ve been doing triathlons for about five years,” she said.
“I have always done those things in small doses, and I decided to put it all together and have a go.
“I had a go for a few years then started doing the local ones, then I thought I’d do some qualifiers.
“This is the first year I have competed for the GB team.
“I felt great. I hadn’t expected to come second, but I did hope to come in the top six. I know what the British competition is like, but I didn’t know what the others were like, like Australia and America. I was more than pleased.”
Both Mrs Derrick, and her daughter Kate, 35, who came fourth at the European Championship in Kitzbühel, Austria, have qualified for next year’s European Triathlon Championships in Glasgow.
Mrs Derrick said: “It is quite nice to go to the same place at the same time.”