Labour's Albert Owen has held Ynys Mon, which had been the most marginal seat in north west Wales.
He had been challenged by former Plaid Cymru leader Ieuan Wyn Jones, who came third after the Conservatives.
Elsewhere Plaid's Hywel Williams narrowly retained Arfon by less than 100 votes and colleague Liz Saville Roberts held Dwyfor Meirionnydd.
The Tories' Guto Bebb held on to Aberconwy.
Mr Owen, who had his biggest majority since he was first elected in 2001, said: "I think there's been a miscalculation by the prime minister and she's misread the mood of the country and the mood was reflected here on Anglesey by people who wanted to talk about the issues of health, education, the economy and various other things."
He said a number of young people had registered with Labour for the first time wanting to help with their campaign.
"One of the things I'm going to do as the MP now is to get a youth forum and crystallise that enthusiasm... so young people's voices get heard loud and clear in Westminster," he said.
Speaking about his Arfon win by a small majority, Hywel Williams said: "That's what happens when you have a presidential election run by both large parties on a May versus Corbyn basis. The third party gets squeezed.
"What's significant about this seat is that we won, with a small majority but our vote remains stable."
Mr Bebb said he was relieved to hold Aberconwy, with a 635 majority over Labour.
"I'll be perfectly frank, I never thought we were going to have the 10% lead that YouGov predicted at the start of this campaign," he said, "but I also never expected to be looking at such a tight results in Aberconwy and losses in other parts of Wales so it's really a disappointing night for the Conservatives in Wales."
He said it was premature to talk about mistakes but said something quite complex and interesting had happened.
"I think there's something more fundamental going on," he added.