The Independent Police Complaints Commission (IPCC) said Jacqueline Oakes, of Birmingham, was let down by a raft of procedural errors and failings to identify the risk she faced.
She was battered to death in 2014.
The West Midlands force said it agreed with the findings on its employees.
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In all, the actions of 40 staff were examined by the IPCC - "one of the biggest investigations" it has carried out, it said.
The inquiry looked into 19 reported incidents involving Ms Oakes, 51, and her killer Marcus Musgrove in the run-up to her death at her home on 14 January 2014.
Musgrove was jailed for life that year.
Errors were made by officers and control room staff, the IPCC said.
Among the failures, the investigation found, were those of five detectives who did not ensure they were adequately informed before determining that Musgrove should be bailed.
On 10 January 2014, four days before she died, Ms Oakes gave a police statement about an incident in which Musgrove breached his bail conditions.
There was an outstanding warrant for Musgrove for the breach when she died.
The IPCC said it found there were at the time "significant organisational failings" in the way the force managed outstanding arrests of this type, which meant "arrest attempts" were not made until 12 January 2014.
Ms Oakes's daughter, Jenny, 36, said: "I will never be able to understand how such a slack attitude to a situation of potential escalating domestic abuse can be taken by so many people at a police force. It is shocking."
In addition to the 19 employees said to have a case to answer for misconduct, a now retired officer was found to have a case to answer for gross misconduct.
No misconduct was identified for a further 20 employees whose actions were investigated.
The IPCC said two officers would attend misconduct meetings, with others subjected to "management action to address the failings".
No action is to be taken against the retired officer.
Deputy Chief Constable Louisa Rolfe, of West Midlands Police, said: "We acknowledge that some of our officers and staff failed in their duty to complete certain checks, risk assessments and, on occasion, follow the force's domestic abuse policy and for this we sincerely apologise to the family of Jacqueline Oakes."

The privy at Brodsworth Hall, near Doncaster, South Yorkshire, was originally built in the 1860s.
In its heyday, its contents, dubbed "night soil", were emptied daily and spread on the gardens by staff.
English Heritage, which now runs the site, said the privy had been restored to its former glory.
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English Heritage's Kendra Grahame-Clarke said the privy was restored using archival information found at the hall, and is now "one of the poshest outdoor privies in the country".
She said although the facility does not have running water, it features a hole under the seat which can be accessed by a trapdoor allowing "a servant/gardener to collect and spread the night soil at dusk".
The toilet is in its own "secret garden" and also features a pagoda-style roof and pergola trellis.
The original toilet, built by George Ball for three pounds and five shillings, was surrounded by heavily scented blooms, and provided a respite for members of the Thellusson family, who lived at the house.
Nineteen police officers and staff have a case to answer for misconduct in their dealings with a domestic violence victim before her murder, a major investigation has found.
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An outdoor privy is being hailed as "one of the poshest in the country" after being restored at a Victorian country house.