The terminally ill 56-year-old from Green Hammerton, near Harrogate, completed the challenge in his wheelchair last week to raise funds for Leeds Hospitals Charity and the MND Association's appeal to build the Rob Burrow Centre for MND.
Footage shared on the father of two's Twitter account showed the moment his support team helped him to the summit of the Welsh peak.
He has now raised over £9,000 - well above his donation target of £5,000.
Mr Flatt is treated at the same specialist MND facility in Seacroft, east Leeds, as former Leeds Rhinos player Rob Burrow, who sent him a congratulatory video message after the feat.
Mr Flatt used an electric off-road wheelchair for the climb and was accompanied by some of his nurses.
Mr Flatt, who has a wife and two daughters, was diagnosed with motor neurone disease in 2019.
He said: "Like all families living with MND we have faced many mountains, which fortunately for us have been navigated with the gentle guidance and support of the MND Team in Leeds and the MND Association.
"And this is where you come in, we really need your help. We would be incredibly grateful for each and every donation to help us raise much needed funds for a dedicated centre that provides respite, research, resources and overall a sense of hope for the MND community and for the exceptional and extraordinary people who look after us, who were there at the beginning of our journey and I know will be there at the end.