The party was hammered at the 2021 state election, with Ms Mettam and former state leader David Honey being the only Liberal MPs left in the lower house.
Ms Mettam challenged for the leadership and took on the job on January 30 after Mr Honey declined to contest the position.
Ms Di Russo, after her election at the party's state council meeting on Saturday, said she looked forward to working closely with Ms Mettam and Ms Cash in preparation for the 2025 state and federal elections.
"Our timeless Liberal values remain fit for purpose. They're the answer to many of the challenges we must face as a country, and the key to unlocking the enormous potential of our great state and the people within it," she said in a statement.
"But only by electing more Liberal voices to parliament can our values have that positive impact."
Ms Cash said Ms Di Russo's election as state president was well deserved.
"I know she has the energy and intellect to successfully lead the Liberal Party of Western Australia, as president, for the crucial years ahead of us.
"This is an historic moment for the party and I'm proud to be part of the first all-women leadership team - a team put in place purely on merit."
Ms Mettam said Ms Di Russo's drive, commitment and staunch Liberal values would ensure the party went from strength to strength in coming years.
