The 48-year-old is only the third person to complete the arduous 18-mile trek, a feat that she said she would never repeat, shortly after arriving home yesterday morning for a shower and some sleep.
Tréhiou, who has completed five Channel swims – one a double-crossing in 2013, said: ‘It was a brutal swim, the second toughest after my double Channel, I really had to push hard from start to finish because of the tide.
‘I would not recommend anyone doing it that has not done a Channel swim, simply because it was such a mental challenge as well.
‘It was a real head-spinner, I had to swim towards Sark first it felt like I was treading water for two hours because I didn’t seem to be going anywhere.
‘It took five hours to get Sark out of sight and when it got dark the jellyfish came out to play; I don’t know how many times I’ve been stung but I’ve got at least eight stings on my face.
‘They were very big, they were wrapping tentacles around my neck and arms and one went down my swimsuit and I had to pull it out.
‘The hardest part was going across to Sark because I just didn’t seem to be going anywhere, and the finish was also really tough as well because because I had to swim hard to get to the rocks [at Plémont].’
Asked what her plans are next, Tréhiou said: ‘There is no plan, it was a tough and long swim and I’m just going to enjoy the moment.’
Nine Guernsey to Jersey swims attempts have been made over the years and only three completed, the previous two by Jersey’s Neil Faudemer, three years ago, and Lancashire-born Ruth Oldham in 1962.
Faudemer was boat pilot for Tréhiou’s swim, aboard Corbière Star, and the super effort started at Guernsey’s St Martin’s Point.
Faudemer said: ‘It was a really difficult swim, more difficult than one would imagine.
‘It was touch and go, but Wendy is a very determined lady and I’m delighted that she completed it.
‘It was a very courageous swim as well, because there were lots of very large jellyfish around and she was stung many times.
‘I looked over the side of the boat and I’ve never seen so many jellyfish and I was thinking that I’d never jump in there!
‘Sea conditions were really good and the sunset on Sunday night and the sunrise yesterday morning were just amazing.’
Tréhiou, who is a cancer survivor and the Macmillan Jersey Ambassador, was raising money for the charity.
Supporting the swimmer on Faudemer’s boat were Sarah Pierce, Graeme Lowe, Simon O’Donoghue and Shirley Lancefield.