Roger Cook has pledged to lead a prosperous, inclusive and fair Western Australia as the state's 31st premier.
Mr Cook on Thursday joined his deputy Rita Saffioti and other cabinet members to swear in at Government House in Perth.
The 57-year-old replaces the retiring Mark McGowan, after serving as his deputy for six years after Labor took office in 2017.
Mr Cook was also health minister at the height of the COVID-19 pandemic before assuming the state development, jobs and trade portfolios.
He said he was honoured to be sworn in by Governor Chris Dawson as premier, describing it as a life undertaking, a responsibility and a privilege.
The new premier earlier shared a photograph on social media of himself with his "incredibly proud" parents Hugh and Lynne, saying they were unable to attend the swearing-in ceremony but would be in his thoughts.
"It's because of them that I'm the man I am today," he wrote.
"As a child, sitting around the family dinner table with my six brothers and sisters - it was those conversations that shaped me.
"They gave me the principles and values I still hold today, which will guide the choices I make as premier for a prosperous, inclusive and fair Western Australia."
A member of Labor's left faction with a background working for Aboriginal land councils, Mr Cook was quickly able to secure support from across the party despite his own union initially backing Amber-Jade Sanderson for the leadership.
On Wednesday, he confirmed Ms Sanderson would retain the health portfolio while Ms Saffioti would juggle the key roles of treasurer and transport minister.
Ms Saffioti will shed planning and ports, the latter to be overseen by sole newcomer David Michael who also takes on local government and road safety.
Police Minister Paul Papalia has added the corrective services portfolio to his responsibilities, replacing Bill Johnston after a tumultuous period of riots and claims of human rights violations in youth detention.
Mr Johnston remains minister for energy, adding the hydrogen industry portfolio.
The WA Liberals panned the minor reshuffle as a lost opportunity for a genuine reset following Mr McGowan's surprise retirement.
A by-election will be held for Mr McGowan's ultra-safe seat of Rockingham in coming weeks after he called time on 27 years in parliament.
