Frean had been based in the UK as a business columnist since 2016, but also spent seven years with the Times in the US as New York business editor and Washington bureau chief.
Her focus in her latter role was technology and diversity.
She has now joined Starling, which was granted a banking licence by the UK's Financial Conduct Authority in July 2016 and launched its current account in May 2017. It has recently added a business account to its product range and has around 170 staff.
The bank has used Pagefield for press office support since September, having worked with the agency on a project basis in 2016 when it secured the FCA licence.
Anne Boden, chief executive of Starling, said: "Alex is a seasoned journalist with an unusually broad range of expertise and interests. She will help drive our corporate affairs team at a critical time in our development as we become a major force in banking."
The business editor of Frean's former stablemate the Sunday Times has also this week announced a move into PR, with Edelman.