She was elevated to the role of county Tory leader on Sunday afternoon, just 24 hours after the party comfortably retained control of the authority in the local elections.
The position in charge of the Conservative group - and hence also the county council itself, in the wake of the election result - was vacant after Geoff Driver, who has led the authority for the past four years, stood down at last week’s poll.
County Cllr Williamson's official instatement as leader of the council will require a confirmatory vote of all of the authority's members at a meeting later this month.
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It had been thought that deputy leader Keith Iddon was being primed to take over the reigns, after spending recent months working closely with County Cllr Driver on the running of the authority. He had been in the deputy role for two years and also featured heavily in the Tories’ pre-election media appearances, representing the party in a Local Democracy Reporting Service debate prior to the county vote.
However, as Saturday’s midday deadline for nominations passed, it became clear that any notion of a coronation had turned into a competition when both he and County Cllr Williamson made bids to become the County Hall boss. A secret ballot of the now 48 members of the Conservative group on the authority came back in favour of the latter.
Responding to the news, County Cllr Williamson said she was "honoured" that her colleagues had "put their faith in me to lead them".
"I am looking forward to the challenge and getting started straight away, delivering better services, inward investment, jobs and opportunities for the county," she said.
On the election result, County Cllr Williamson described it as "reflecting the hard work of our councillors over the last four years".
"We are pleased that voters have trusted us to deliver for them. We want to build on that trust and, with our new, expanded group of talented councillors, to continue that work, " she added.
County Cllr Williamson, whose CV includes four years as chief executive of the Serious Fraud Office, was first elected to the authority in 2017 and represents the Lancaster Rural North division.
She was promoted to cabinet as the member for children, young people and schools two years later and has since overseen the introduction of a new family safeguarding model designed to reduce the number of children going into care.
Her deputy will be Alan Vincent, who is also currently the deputy leader of Wyre Council. No details of any other changes to the county's cabinet have yet been revealed.
County Cllr Iddon, who has also held the high-profile role of transport and highways cabinet member on the county council for the last four years, said he had enjoyed his “moment in the sun”.
“I wish Phillippa and the group well and I hope they go from strength to strength,” said County Cllr Iddon, who was re-elected on Saturday to the Chorley Rural West division that he has served since 2009.