It comes as the British Dental Association call for "radical and urgent change" to help NHS dentistry recover from the impact of Covid-19.
Figures from the NHS show 169,970 adults were seen by an NHS dentist in Cornwall in the two years leading to June 30 this year, 39% of the area's adult population.
It is up from 37% in 2020-22 but below the rate before Covid-19 when 52% were seen in 2017-19.
Nationally, 18.1 million adults were seen by an NHS dentist in the 24 months to June 30 this year. It is up from 16.4 million in 2020-2022, but still below the 22 million seen in 2017-19.
BDA chairman Eddie Crouch said: "We’re seeing the limits on the recovery and this Government’s ambition.
"Demoralised dentists are walking away from a broken system, while millions struggle to access the care they need. NHS dentistry can come back from the brink, but only if ministers turn the page."
The BDA dismissed the Government's changes to the NHS dentistry contract at the start of the current financial year as "minor tweaks".
An NHS spokesperson said the figures show significant recovery in dental treatment for children since the pandemic with 6.4 million seen in the year to June 2023.
In Cornwall, 51,996 children were seen in the past year – covering 51% of under-18s in the area.
The rate is up from 46% in 2021-22 but below 62% in 2018-19.
Louise Ansari, chief executive at Healthwatch England, said the new data backs up its claims "that people in every corner of England are struggling to get the dental treatment they need when they need it".
She said: "NHS dentistry continues to be the second most common issue people report to Healthwatch, with many living in pain, while some turn to private care.
"But private treatment is not an option for everyone, with reports suggesting people from the most deprived communities struggle the most to access dental care."
The figures also show the number of NHS dentists practising in England also fell year-on-year to 24,151 in 2022-23 from 24,272 in the previous year.
Ms Ansari added Healthwatch England has repeatedly called for the Government to publish its dental recovery plan.
Earlier this month, the Government said the blueprint is "due to be announced shortly".
A Department of Health and Social Care spokesperson said: "We are making progress to boost NHS dental services with 23% more courses of treatment, meaning 1.7 million more adults and 800,000 more children received NHS dental care.
"We fund more than £3 billion of NHS dentistry a year, have announced plans to increase dental training places by 40% and last week we launched a consultation to better utilise the skills of dental hygienists and therapists."