Thicha Na Nakorn, a national reform councillor on education and constitution drafter, has submitted a resignation from both positions, effective on Sunday.
She posted a photo of her resignation letter on her Facebook on Saturday but could not be reached by phone for comments, Thai media reported.
Mrs Thicha, 63, is the director of Ban Kanjanapisek, a training centre for convicted boys.
She posted she had not wished to apply to join the National Reform Council (NRC) but her "most respected puyai" had asked her to.
"He sent staff to help me with application paperwork, except for the vision statement, which was something I needed to write myself," she wrote.
Mrs Thicha's resignation letter posted on Ban Kanjanapisek Facebook on Feb 28, 2015. (Facebook photos)
When she learned she was among the chosen 250 from some 7,000 candidates for the NRC and had to report herself, she could not find her diploma.
"A reporter called to remind me I was the only one who had not reported for duty and I told her the reason.
"She talked with parliament officials and said I could submit the document at a later date."
On the day she had to give her vision statement to join the Constitution Drafting Committee, she barely made it on time.
"I had to go to court to testify in a case in which I had refused to give some pills to children against the advice of the court and doctors.
"Somehow, I was voted the 12th in the 20-man CDC," she wrote.
"I later learned the salaries I got from these jobs were very high [for me] so I donated 40% of them every month.
"A full four months are enough for 'a fish in the wrong water' like me," she concluded.
Mrs Thicha also thanked those involved and apologised to everyone she might have disappointed.
According to the interim charter, the 36 CDC members come from quotas of the so-called "five rivers" analogised by the junta as the key elements to realising true democracy. The NCPO selected five members plus the CDC chairman, the NRC selected 20 and the National Legislative Assembly and the cabinet chose five each.
Mrs Thicha came from the NRC's quota.
Commenting on Mrs Thicha's resignation on Saturday, CDC chairman Borwornsak Uwanno said the NRC would simply move the person next on the list in its vote when it selected CDC members under its quota on Oct 29 last year.
According to records, the 21st on the list is Nantawat Boramanand, dean of the Faculty of Law, Chulalongkorn University.