Casey grew up in Portsmouth and began her career with the Department of Health and Social Security (DHSS), administering benefit payments for homeless people. She then worked for the St Mungo Association, a charity that helps homeless people. She became director of the Homeless Network in London, before becoming deputy director of Shelter in 1992. At Shelter she gained a reputation as an "ambitious, pragmatic worker who got results" and was largely responsible for the creation in 1998 of Shelterline, the country's first 24-hour telephone helpline for homeless people.
Following the 1997 election, the Labour government in December that year created the Social Exclusion Unit (SEU), which had tackling rough sleeping as one of its priorities. In April 1999 the RSU was created and Casey appointed by Prime Minister Tony Blair as its head, referred to in the media as the "homelessness czar". With an eventual budget of £200 million, the RSU's aim was to reduce the number of rough sleepers in England by two-thirds by April 2002. The RSU published its strategy in December 1999.
Casey triggered some controversy in November 1999 when she said the activities of some charities had the effect of keeping homeless people on the streets: "With soup runs and other kinds of charity help, well-meaning people are spending money servicing the problem on the streets and keeping it there. Even The Big Issue is perpetuating the problem." The editor of The Big Issue criticised her comments.
In November 2000, the government launched the RSU-led "Change a Life" campaign, which encouraged people to give money to homelessness charities instead of to beggars, following research suggesting that 86 percent of beggars used drugs. Casey said giving money to beggars was "misplaced goodwill". The donations hotline set up as part of the campaign was closed in March 2002, having collected £10,000, despite advertising spending of £240,000. The RSU achieved its target in November 2001, several months before the deadline, but allegations were made that they had used underhand tactics; Casey responded that they were false. With her work at the RSU finished, she became director of the newly created Homelessness Directorate.
In January 2003, Casey became head of the ASBU at the Home Office. Introduced in 1998, an Anti-Social Behaviour Order is a civil order made against a person who has been shown, on the balance of evidence, to have engaged in anti-social behaviour.
During an after-dinner speech at a private (Home Office/ACPO) function in June 2005, she said that ministers would perform better if they were "pissed" and, "doing things sober is no way to get things done". She added, "There is an obsession with evidence-based policy. If No 10 says bloody evidence-based policy to me one more time I'll deck them one and probably get unemployed." The remarks, defended by the Prime Minister's office, led to an inquiry, after which Casey, having apologised, remained at her position.
In September 2005, Casey was appointed head of the Respect Task Force as part of Blair's "respect agenda", becoming known as the "respect czar". The Respect Action Plan, launched in January 2006, was designed to deal with anti-social behaviour and problematic young people and families. In December 2007, the task force was closed down, and Casey moved to another job involving community policing. Her review of "Engaging Communities in Fighting Crime" was published in 2008, being based largely on her contact with the public. In June 2008, Casey was made a Companion of the Order of the Bath. She recommended the requirement, introduced in December 2008, that offenders doing community work should wear fluorescent orange jackets with the words "community payback" printed on the backs. In October 2009, while working as the government's neighbourhood crime adviser, Casey said that the justice system favoured criminals, and the public wanted a justice system that was not a "criminal's justice system".
On 30 March 2010, Casey was appointed to the post of Commissioner for Victims and Witnesses, created under the Domestic Violence, Crime and Victims Act 2004, whose objective is to "promote the interests of victims and witnesses, encourage good practice in their treatment, and regularly review the Code of Practice for Victims which sets out the services victims can expect to receive". As Victims' Commissioner, Casey said crime victims were treated poorly by the system, and suggested jury trials were unnecessary for many lesser offences.
It was reported in September 2011 that Casey would work with Prime Minister David Cameron in dealing with the consequences of widespread rioting a month earlier, and she resigned from the position of Victims' Commissioner on 12 October 2011.
She became Director General, Troubled Families on 1 November 2011. The Troubled Families programme initially intends to change the repeating generational patterns of poor parenting, abuse, violence, drug use, anti-social behaviour and crime in the 120,000 most troubled families in the UK, with the government investing some £4,000 per family over 3 years, and each family having an assigned family worker. Mental health problems are often found in such families. In January 2013, the Department for Communities and Local Government (DCLG) commissioned a consortium to provide an independent evaluation of the Phase One Troubled Families Programme.
By November 2013, some 22,000 families had been "turned round", based on metrics such as child school attendance and crimes committed, although at least some of these families continue to commit some crimes.
Casey does not believe people undertake behaviours to gain benefits, and that compulsory contraception, while reducing the number of children being born into such families, would lead to high-risk teens finding "something else to get into trouble with. Because they've got trouble in their souls, trouble in their heart, troubles in their head. So even if you brought in some draconian thing like that, they'd find something else to do that would actually be an expression of not having enough love or of having too much pain".
Casey also told The Daily Telegraph (20.07.12): 'We are not running some cuddly social workers programme...we should be talking about things like shame and guilt...we have lost the ability to be judgmental because we worry about being seen as nasty to poor people'.
In June 2013, the UK government announced its intention to extend this intensive help to 400,000 more families, committing £200 million in funding in 2015 to 2016. It expects, for every £4,000 spent on a family, an annual saving of £15,000 in the costs of the police, health and social services in dealing with the family. In March 2015, Casey and the Department for Communities and Local Government website asserted that 105,671 families of the 117,910 processed had been "turned around", some 89.6%, saving £1.2 billion per annum although Casey had said that the TFP would save £9 billion per annum at the start of the scheme. According to the Department for Communities and Local Government website, each family before programme entry, cost £26,000 per annum. The TFP allegedly saved £11,200 per family implying that 56.9% of the original cost still remained although 89.6% of families had been 'turned around'.
The evaluation by the National Institute for Economic and Social Research was published on 17 October 2016. The report found that there had been "no significant impact" of the scheme. Casey stated: “They (NIESR) had not, frankly, put any of the caveats in the public domain” and that “they have misrepresented their own research”. NIESR disputed these statements.
Following the publication of a report by Alexis Jay on the Rotherham child sexual exploitation scandal, Casey was appointed by Eric Pickles to lead the inspection of the children's services at Rotherham council. The Guardian reported on 10 September 2014: "In his written ministerial statement, Pickles says he has directed Casey to consider how the council exercised its functions on governance, children and young people, and taxi and private hire licensing.'"
Casey's report was published on 4 February 2015, and found that the local authority's child sexual exploitation (CSE) team was poorly directed, suffered from excessive case loads, and did not share information. The Secretary of State for Communities and Local Government, Eric Pickles, said that the local authority was "not fit for purpose", and announced proposals to remove its control from the councillors and give it to a team of five appointed commissioners. The Labour Party leader Ed Miliband said that his party had "let people down in Rotherham". However, Casey's report was also heavily criticised by social work academics in Community Care in March 2015:
"There are troubling aspects of the report...the process by which it was prepared, in particular the lack of rigor and transparency in the methods used to gather and analyse data...This gap [in methodology] ...should concern us as it goes to the heart of issues of accuracy."
After the end of the Rotherham report, Casey was asked to undertake a broader review of community cohesion and extremism. In October 2016, some newspapers printed that the report had been ready for months but had not been published because of concerns at the Home Office on its content with regards to immigration. The report apparently criticised the Home Office for a lack of strategy to integrate new immigrants into communities and to respond to extremism amongst Muslims.
The Review was finally published on 5 December 2016. The Review stated segregation and social exclusion are at "worrying levels" and are fuelling inequality in some areas of Britain. Women in some communities are denied "even their basic rights as British residents". The report described the plight of women in some Muslim communities, who were "less likely to speak English and more likely to be kept at home". Among Casey's recommendations were that immigrants could take "an oath of integration with British values and society" and schoolchildren be taught about British values.
Casey was appointed Companion of the Order of the Bath (CB) in the 2008 Birthday Honours and Dame Commander of the Order of the British Empire (DBE) in the 2016 Birthday Honours for services to families and vulnerable people.
In February 2013, she was assessed as one of the 100 most powerful women in the United Kingdom by Woman's Hour on BBC Radio 4.