GPs working in the NHS Buckinghamshire Clinical Commissioning Group area issued a total of 61,761 notes between October 2017 and September 2018.
People in work need a fit note, previously called a sick note, if they are off work for more than seven days. Up to that point they can self-certify that they are unwell.
Mental health is the most significant single cause. Of the notes signed in Buckinghamshire, 7,902 or 13% were issued for mental health problems. A further 4,628 were the result of musculoskeletal conditions such as back problems.
Research by mental health charity Mind found that nearly half of workers said they had suffered from mental health issues during their current employment.
Emma Mamo, head of workplace wellbeing at the charity, said: "Mental health problems can be debilitating and should be seen as a valid reason for needing time off.
"However, if staff are well supported they can and do make a valuable contribution to the workplace, whether they have a mental health problem or not.
“Thankfully, many employers are making strides when it comes to prioritising workplace wellbeing, recognising well-supported workers are happier, more productive and loyal."
Across England, 5.6 million fit notes were handed out over the 12 months to the end of September last year.
The number of fit notes signed in Buckinghamshire in that period is up on the previous 12 months despite under-reporting last February caused by data-gathering issues.
Doctors in the area issued 1,654 fit notes for every 100,000 people on their registers, on average, each month. The highest rate was 3,641 per 100,000 in Halton and lowest rate was 867 in Richmond.
Despite being called fit notes, almost all of those issued classify people as unfit for work. This was the case for 94% of the notes handed out across the country.
However, fit notes are also used to advise patients and their employers on getting back to work with reduced hours or limited duties.
Women were signed off sick a little more than men. They received 57% of the fit notes across England over the 12 months.