Grinnell succeeds Louise Shepherd, who departed last month following 16 years in charge.
He joined the trust in April 2017 as director of finance and deputy chief executive. More recently, in April 2023, John took on an enhanced leadership role as managing director and chief finance officer, driving development of the trust's ambitious new strategy to deliver a healthier, happier, and fairer future for children and young people.
Grinnell has nearly 30 years of experience in the NHS and has worked for 17 years at board level.
He will assume his new role immediately, having undertaken the duties of Interim Chief Executive since Louise's departure.
Dame Jo Williams, Chair of Alder Hey Children's NHS Foundation Trust, said: 'John is already well known – and well respected – within Alder Hey, and our wider, local health economy, having played a leading role since joining us in April 2017.
'John's expertise and experience of managing NHS finances is well recognised, but it is matched by his absolute commitment to ensuring children and young people have the best possible start to life.
'His skills and vision are exactly what Alder Hey needs, and I'm confident that with the support of the Trust Board, the Executive team, and our colleagues, he will successfully guide us through the next stage of our journey.'
Grinnell added: ‘It's a deep honour to have been entrusted with the care of Alder Hey. It's a responsibility I take very seriously because I recognise the important role that we, as part of a wider healthcare system, must play in addressing the health inequalities that exist in our City Region and beyond. Working alongside our brilliant people and partners, I am confident that we can make a difference.'