He has so far raised more than £3,500, including Gift Aid, after initially setting a £420 target.
The money was raised to help Malmesbury and District Foodbank, which provides emergency food parcels to those in food poverty.
Mr Groome, 46, of Lea, said: “This started as an idea to push myself and to try and raise £1,000 for Malmesbury Foodbank.
“Thanks to the generosity of those who supported me we have now raised over £3,500, the largest donation ever received by the foodbank.
“The team there would like to express their deepest gratitude for taking the time to help at a time when their services are in high demand.”
“I did a full Ironman Triathlon in my garden because I wanted to make a difference.
“This involved me swimming in a paddling pool for about 1.5 hours, cycling 112 miles on a static Wattbike and then running 26.2 miles, which was around 420 laps of my garden.
“For the swim I was tethered so was static... think of it as 1.5 hours of front crawl but never being able to reach the end of the pool.”
Mr Groome started triathlon at 7am and finished by 7pm.
The Malmesbury and District Foodbank relies entirely on donations, grants and gifts.
It is a registered charity and part of the Trussell Trust network of nationwide foodbanks.
Mr Groome added: "I had no idea what a difference it would make and look forward to being able to help the foodbank in the future too.”
To donate to Mr Groome's fundraising appeal for the foodbank, go to https://www.gofundme.com/f/gardenironman