McAllister will take up the role on 1 October until 1 March next year unless a permanent post-holder is appointed before then.
During her time as ombudsman between 2018 and 2022, McAllister led two independent investigations into deaths of babies in custody.
Her report on the death of Baby A at HMP Bronzefield was published in September 2021, and the report on the death of Baby B at HMP Styal was published in January 2022.
Prior to becoming PPO, McAllister was the director of reducing offending and prison service director general at the Department of Justice in Northern Ireland.
She has held senior policy roles including head of the public sector bids unit at the National Offender Management Service and worked as prison governor, head of security group and area manager for the West Midlands.
McAllister also led an independent investigation into a serious disturbance at an immigration removal centre for the Home Office.
Russell announced he would step down earlier this year after taking on the role in June 2019.
On announcing his departure he paid tribute the “resilience, compassion, commitment, and imagination” of those working in the youth justice system.
