Former PC Mills was found to have breached standards of professional beahviour at a misconduct hearing held at Sussex Police headquarters in Lewes on Friday (May 10).
He would have been dismissed from the force without notice had he not already resigned earlier this month, police said.
The independently-chaired hearing heard in July 2016 former PC Mills had failed to adequately investigate allegations of harassment and stalking in the case of Shana Grice.
He had further failed to respond to a report of harassment and stalking made by Shana Grice on July 12, 2016, not contacting her or updating her on the incident. As a result, she was not treated as a victim of domestic abuse, police said.
The hearing panel found both allegations were proved and amounted to breaches of standards of professional behaviour in respect of duties and responsibilities; equality and diversity and discreditable conduct.
Former PC Mills did not attend, but the panel judged if he had still been serving, he would have been dismissed without notice.
Assistant Chief Constable Nick May, said, “Mr Mills’s actions were clearly completely out of keeping with his role that others uphold with pride, integrity and with the trust of the public whom they serve.
“We deeply regret the tragic death of Shana Grice in 2016 and are committed to constantly improving our understanding of stalking and our response to it.
“Our then deputy chief constable personally visited Shana’s family to apologise on behalf of Sussex Police.
“When we looked at the circumstances leading to Shana’s murder, we felt we may not have done the very best we could and made a referral to the Independent Office for Police Conduct (IOPC).
“Since then we have undertaken all their recommendations, thoroughly reviewed all aspects of how we deal with cases of stalking and harassment and have significantly improved our service to victims.
“We encourage victims to come forward with the knowledge that our officers and staff are better trained and that they will take all reports seriously. We are absolutely aware of the consequences if our response is not the correct one, so we want to ensure that victims have confidence in how both police and the CPS will support them.”
He continued, “Sussex Police expects the highest personal and professional standards of anyone who works for us and any allegations of behaviour that do not meet those standards are rigorously investigated. Police officers must behave in a manner that does not discredit the police service or undermine public confidence.”
A further misconduct hearing for an officer who has now retired will take place at a date to be set.
Another police officer is to face internal misconduct proceedings on May 17. Three other police officers and three members of police staff have received management advice and further training, said Sussex Police. Five other police and staff are not to face any action.