The patient, who had physical disabilities as well as learning difficulties and autism, complained of vomiting and headaches but was given painkillers by GPs and out-of-hours doctors at NHS Highland.
She died six days before she was due to have an MRI scan in September 2014.
A hard-hitting report found that the patient could have had a good chance of survival if the tumour had been spotted earlier.
Scotland’s ombudsman Jim Martin ordered significant event reviews at both the GP practice and Raigmore Hospital, in Inverness, due to the seriousness of the case and told doctors to apologise to Ms A’s family.
NHS Highland said it was “truly sorry” for the care the patient received and pledged to carry out a probe into the incident.
A report by the watchdog found Ms A suffered “significant personal injustice” as both the GP practice and the health board failed to offer her appropriate treatment.
It said: “I have outlined the advice from Adviser 5, who considered that, had the seriousness of Ms A’s symptoms been recognised, and her tumour detected
earlier, she could have been given surgery, with a very good chance of success.
“Ms C has described the deep distress of Ms A’s family at the untimely death of their daughter, and the advice I have received indicates that the failings in care contributed to this tragic outcome.”
The patient began complaining of headaches in December 2013 and underwent surgery two months later to remove a nasal ulcer, the report said.
She returned for follow-up appointments but doctors did not find anything to cause concern.
However in July 2014 she returned to her GP a number of times with persistent headaches and vomiting, and she was seen by out-of-hours doctors four times in 10 days.
Hospital doctors diagnosed her with occipital nerve compression and sent her home again but the symptoms worsened until she was having shaking episodes and struggling to walk, the report said.
She was referred for an MRI by her GP but died before it could be carried out. A post mortem revealed she had a brain tumour in her central nervous system.
The report found that GP’s failed to pick up on the ‘red flag’ signs for a brain tumour and out-of-hours doctors did not seek expert guidance or carry out a full neurological assessment.
An NHS Highland spokesperson said: “We are truly sorry for the standards in the care and treatment provided to this patient and will be writing to the family offering our sincere apologies.
“We accept the findings of the report and it has been shared with staff and senior managers. We will also be conducting a significant adverse event review which will be chaired by a senior doctor.
“This will involve analysing the clinical care and treatment provided in order to learn and implement improvements in our practice. The family will also be kept up to date on this process.”