She had been charged with murdering her son Ryan and attempting to kill her mother at her home in April 2007.
In a narrative verdict, the inquest jury said Ms Manser's death was "suicide beyond all reasonable doubt".
She died from asphyxia, caused by a self-applied ligature.
The jury said staff shortages meant she had not been observed in her room every five minutes as she was supposed to be.
The verdict continued: "The cumulative effect resulted in Emma Manser having the opportunity and time to end her life."
The jury at Haywards Heath Magistrates' Court had heard Ms Manser had a history of mental health problems.
It looks like the protection of my daughter from herself was not adequate on that day Robert Manser Jurors were told that although she could have taken her life in two minutes regular staff observations of her room every five minutes were not strictly observed.
The inquest heard The Dene was also short-staffed on the day she committed suicide, with five members on duty.
Acting ward manager, Ronald McClung, said despite the shortages he did not think the ward was unsafe.
A note was found in her room at The Dene, which read: "I'm so sorry for what I have done and I cannot live knowing I killed the best thing about me. I just can't go on living without Ryan."
Two days before her death, her father's partner received a card from Ms Manser which expressed her sense of loss and grief.
Following the verdict, Robert Manser, said generally staff looked after his daughter's wellbeing, but added: "It looks like the protection of my daughter from herself was not adequate on that day.
"I think the ward was understaffed. When Emma was well, she was a loving daughter and mum, and a joy to be with."