Chief Constable Dave Thompson said he was "deeply sorry" the force had failed to stop Leroy Campbell committing the brutal murder of Lisa Skidmore.
Campbell was not recalled to prison despite glaring warning signs that he could be about to commit a serious crime.
The then 55-year-old convicted sex offender had confessed to probation officers that he was thinking about raping again and was "seeing open windows" just weeks before he climbed through the window of the 37-year-old's Bilston home and attacked her.
Miss Skidmore's elderly mother Margaret was also attacked and the house set on fire.
Mr Thompson, the West Midlands' most senior officer, addressed her family during a meeting at the force's Lloyd House headquarters in Birmingham today, admitting the police and probation service had "failed" and that her murder "should not have happened".
Miss Skidmore's brother and sister, however, said the apology had come "too late".
An inquest jury ruled last year that failings by agencies were “more than a minimal contribution” to Miss Skidmore's death in November 2016.
The force said a series of changes had been made, including closer ties with the probation service, in an attempt to make sure no other serious offenders are allowed to slip through the net.
Addressing Miss Skidmore's family, Mr Thompson said: "I have seen some very sad days in my 29 years as a police officer and the day Lisa was murdered and her attacker identified were amongst the saddest.
"West Midlands Police and the Probation Service are here to protect the public and it is clear in this case that didn't happen. Lisa should not have been murdered or her mother attacked.
"I want to say to you I am sorry we as West Midlands Police failed to protect your family. I am deeply sorry.
"Nothing can change what happened but what we are talking about today will ensure the police and Probation Service work together to prevent cases like this from happening."
Police and Crime Commissioner David Jamieson said it was "the most shocking case in very many years".
"The stress that must have been caused to you must be ongoing and enormous," he added.
Campbell will now never be released from prison, having been given a whole life sentence in May 2017.