Cllr Beca Roberts, 30, is the youngest chair ever of Gwynedd Council.
She and new vice-chair, Cllr Ioan Thomas were congratulated by retiring chair, Cllr R Medwyn Hughes.
The chair and vice-chair received their chains of office at the start of 9 May’s full council meeting.
Cllr Hughes’ highlights as chair include opening a new attraction at Zip World, Aberdyfi’s flood prevention scheme, the introduction of green buses and welcoming the National Eisteddfodd to Gwynedd.
He thanked assistants and colleagues for their support, saying: “I could not have done it without you.”
He also thanked the council members who “allowed” him to be there and for their “good collaboration and behaviour – well at least some of you,” he joked.
“It had been a great honour to represent Cyngor Gwynedd, and I wish Beca well.”
Cllr Roberts thanked Cllr Hughes, saying it had been “a great pleasure working with him”.
In a statement, she said: “As a younger person, I believe we have the ability to influence, open doors and encourage political interest among other young people.
Cllr Roberts gained 43 votes against Cllr Beth Lawton, who achieved 19 votes.
Cllr Thomas was elected with 23 votes against Cllr Elfed P Roberts, who achieved 21 votes.