Norman Forde, 73, was originally charged with murder after killing Aileen Bell at their home in Frederick Avenue, Hinckley, Leicestershire.
He was later deemed unfit to plead and a trial of facts was held at Leicester Crown Court instead.
Forde has been sectioned under the Mental Health Act. Police have not said what kind of mental illness he has.
Det Ch Insp Natalee Wignall, from Leicestershire Police, said: "This tragic incident has been extremely upsetting for those close to Aileen, and they continue to come to terms with the events of that night."
Police said Forde stabbed his 60-year-old partner multiple times following an argument at their home in April 2015, leaving her fatally injured.
A post-mortem examination confirmed she died from blood loss as a result of stab wounds.
Neighbours told the Hinckley Times although the couple never married they had children and grandchildren.
One said: "Norman was the most chilled-out guy. He used to wander past and we'd always talk and pass the time of day."
Forde cannot be released from hospital unless the Ministry of Justice or a mental health tribunal gives consent.
A man has been detained in hospital after being found to have stabbed his long-term partner to death.