The leadership battle was triggered on Friday after Opposition leader Mia Davies announced her resignation.
Ms Mettam said the WA Liberals had not been as effective as they should have been and an urgent change was needed them to be competitive.
It came at partyroom meeting on Monday where Mr Honey declined to contest a vote for the leadership after previously saying he would fight to keep his job.
"I am honoured to have been chosen by my colleagues as the new leader of the WA Liberal Party," Ms Mettam said in a statement after the meeting.
"Every day I will fight to get the best deal for WA."
Ms Mettam thanked Mr Honey, the member for Cottesloe, for his hard work and dedication.
"I look forward to continuing to work closely with you," she said.
The pair are WA's only Liberal MPs in the lower house after the party was hammered at the 2021 state election.
The Nationals won four seats, and the two parties formed an alliance with Ms Davies as Opposition leader.
The Nationals are expected to announce a new Opposition leader to replace Ms Davies after a partyroom meeting on Monday.
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