The son of late Labour politician Carl Sargeant thanked well-wishers from across the world for standing by his family during their "toughest time" as he succeeded his father in the Welsh Assembly.
After he seized victory, Jack Sargeant said now was not the time for questions to be answered over the events that led to the by-election taking place in Alyn and Deeside.
Carl Sargeant had been suspended from the party and dismissed as a Welsh Assembly minister following unspecified allegations about his personal conduct.
An independent QC-led inquiry has been launched into how first minister Carywn Jones handled the sacking of Sargeant, while an inquest into the late minister's apparent suicide four days later at his home in Connah's Quay, Flintshire, has been opened and adjourned.
At the start of his victory speech, 23-year-old Jack Sargeant gave personal thanks to his mother, Bernie, and sister Lucy, who both attended the count in Connah's Quay, and the rest of his family and friends who "have supported me day in day out, not just during this campaign but forever."
He went on: "Above all, I want to say a big thankyou to every person standing in this room tonight, everyone across Wales, across the UK and across the world as well who have stood by my family during the toughest time of our lives.
"I really can't bring together the words to say thank you enough but it just means so much to me and I know this special community in Alyn and Deeside have helped all of us so much, and I don't think we would get that anywhere else.
"There are questions still to be answered surrounding why we are standing for this by-election and there are inquiries underway, but tonight is not the night for those questions to be answered. That will be done in due course.
"Tonight is about celebrating a victory on behalf of the people of Alyn and Deeside. So now it's my turn to stand alongside the people of Alyn and Deeside like they have stood alongside myself and the family and all close friends."
He urged a return to traditional Labour values of looking after those who are unable to do so themselves and added: "Let's be kind to each other because that's the message I learn from my dad."
Sargeant increased Labour's vote share by nearly 15 percentage points on that achieved by his father at the 2016 Welsh Assembly election, up from 45.73% to 60.65%.
There was a swing of more than 5% from Conservative to Labour, even though the Tories increased their vote share by more than four percentage points.