Longfield will lead a three-person panel at the helm of the inquiry, which will direct local investigations into how agencies including children's social care and the police failed victims of group-based child sexual exploitation (CSE), while also examining national issues, including the role of government.
The peer has resigned from the Labour Party group in the House of Lords to take up the post, and will serve alongside Zoë Billingham, previously an inspector at Her Majesty's Inspectorate of Constabulary and Fire & Rescue Services, and former council chief executive Eleanor Kelly.
Following a huge political row over the issue of group-based CSE at the turn of the year, the government initially resisted calls to set up a public inquiry into the role of agencies in tackling group-based CSE, which was considered during the 2015-22 Independent Inquiry into Child Sexual Abuse.
Louise Casey's recommendations
Instead, it commissioned former senior civil servant Baroness (Louise) Casey to conduct an audit to examine the current scale and nature of the problem and make recommendations to address present and historic failures by agencies.
However, when she reported in June this year, Casey recommended the establishment of a public inquiry to set up and direct local investigations in areas where agencies had failed to protect victims and survivors, a recommendation that the government accepted.
There followed a further row over who should chair the inquiry, with some victims and survivors resigning from a liaison panel over the Home Office's handling of the process, and two candidates - former director of children's services Annie Hudson and ex-police safeguarding lead Jim Gamble - withdrawing their names from consideration.
Longfield has been appointed on the recommendation of Casey, who will act as adviser to the inquiry. She said: "Baroness Longfield, Zoë Billingham and Eleanor Kelly have long-standing track records in advocating for children, holding police forces to account and leading on critical social issues. Together, they make a formidable team and have my full support."
Inquiry 'will not shy away from uncomfortable truths'
Longfield herself said: "The inquiry owes it to the victims, survivors and the wider public to identify the truth, address past failings and ensure that children and young people today are protected in a way that others were not.
"The inquiry will follow the evidence and will not shy away from difficult or uncomfortable truths wherever we find them."
The inquiry, which will have the power to compel witnesses to give evidence and require organisations to hand over documents and records, has been backed by £65m and will last no longer than three years.
Alongside the appointments of Longfield, Billingham and Kelly, the Home Office published draft terms of reference, which, it said, Longfield would consult on before they were finalised in March next year.
The proposed terms state that the inquiry would cover England and Wales and would cover group-based child sexual exploitation and abuse, where two or more connected perpetrators abuse, or facilitate the abuse of, children.
Local investigations into agency failings
Its purpose would be to "identify and illuminate failings in historic and current practice in tackling grooming gangs", through a series of local investigations and a review of actions and failings at national level, including by government. It would then make local and national recommendations for change.
The focus of local investigations would be on failings in systems and procedures, and by individual leaders, in protecting children from group-based abuse and exploitation, covering agencies including councils, the police, the NHS and local safeguarding partnerships, according to the draft terms.
The Home Office has also proposed that the inquiry would make referrals to relevant professional bodies, as appropriate, for failures to carry out responsibilities.
The inquiry will also collaborate with a national police investigation - Operation Beaconport - which is reviewing hundreds of closed cases of group-based abuse and exploitation to examine whether more alleged perpetrators can be charged.
Considering ethnicity of perpetrators and victims
In her audit, Casey found there had been an "appalling lack of data on ethnicity" in relation to the perpetrators of group-based child sexual exploitation and abuse and that the issue had been "shied away from" by agencies.
Her examination of available data from three police forces found disproportionate numbers of men from Asian ethnic backgrounds among suspects, and said this warranted further examination.
The draft terms of the inquiry state that it should "examine how ethnicity, religion or culture played a role in responses at a local and national level" and consider the ethnicity, culture and religion of perpetrators and victims.
