Mr Clark, 71, from Silverknowes, first became involved with the centre after his sister Isa developed lung cancer, and after she died he decided to start volunteering to help others.
Yesterday, he was presented with a long-service certificate by the Duchess of Rothesay, president of the Maggie's Cancer Caring Centres, at a celebration event at Holyroodhouse.
Mr Clark said: "I first started coming here with my sister Isa when she had lung cancer, and they did so much for her, helping her to feel comfortable and giving her somewhere to go.
"Quite a few people in my family have had cancer over the years, so when she passed away 13 years ago, and I saw that they were looking for volunteers, I decided to go for it.
"I just wanted to give something back, as they really had been invaluable to her."
Mr Clark was born in Granton in 1939, and studied at St David's Primary School and St Anthony's High School.
After finishing school, he said he had "lots of different jobs", but eventually settled into work at the Tennent's Caledonian Brewery, where he spent 27 years.
When the brewery closed down he was offered a good severance deal, and he said it was this that had allowed him to devote so much time to volunteering.
Mr Clark said he planned to continue with the work for as long as possible. "It is a pleasure to do this work, and the staff are all so friendly and helpful," he said.
"They provide a vital service for people with cancer, and I hope I will be volunteering here for several years to come.
"I have met the Duchess before when she came to Edinburgh and she is a very nice lady, so it was very special to get this award from her."
Maggie's inaugural centre opened in Edinburgh in 1996, with the charity growing to 15 centres either established or in development.
Since 1996, Maggie's Centres have received more than 400,000 visits and have developed a programme of support that's recognised as providing exceptional cancer care.
Maggie's Centres CEO Laura Lee said: "I would like to take this opportunity to reflect on the past 15 years and thank everyone who has helped Maggie's, from fundraisers to volunteers to the media.
"I, and the thousands of people whom our centres have supported, are extremely grateful.
"It is an exciting time for Maggie's, and, as we mark our 15th year, I also want to take the time to look ahead with great optimism and hope as we grow our network of centres across the UK."