Poor mental health affects half of all employees, according to a survey of 44,000 people carried out by the mental health charity Mind.
Only half of those who had experienced problems with stress, anxiety or low mood had talked to their employer about it.
That's something that must change, says Mind.
Fear, shame and job insecurity are some of the reasons people may choose to hide their worries.
Natalie Hunt, 34, from Salford, got her first job at 18. That role was working in a department store, serving customers, but she found it extremely stressful.
"It was dealing with complaints and helping people with queries. I'd had anxiety and depression as a teenager and the full-time job made me really anxious. I began to get shy and withdrawn, going more and more into myself, and I was worried about having a panic attack at work.
"Colleagues started to notice and eventually my boss wanted a word."
Natalie says that at the time, her employer didn't really understand or know what to do. There was no support. She then left the workplace altogether and took up an art course at college.
She now teaches art classes to people with mental health problems, and at a homeless shelter. She also works part-time in an office, even though sometimes she can go through stages of poor mental health.
Natalie says it makes a huge difference when the workplace is supportive - they have flexible hours and regular catch-ups.
"I first started back in the workplace with a bit of voluntary work in a charity shop, which was great. Because it was voluntary and part-time, I didn't feel pressured and it helped me regain some confidence. That was when I was 20.
"Now I run my own art classes for people with mental health conditions. It's lovely to be making a difference."
Mind says around 300,000 people lose their job each year due to a mental health problem.
The charity - along with The Royal Foundation, Heads Together and 11 other organisations - has created an online resource for employers and employees with information, advice, resources and training that workplaces can use to improve wellbeing.
A recent poll by the Institute of Directors found less than one in five firms offered mental health training for managers.
Poor relationships with line managers, along with workload, have the biggest negative impact on employees' mental health, the survey found - closely followed by poor relationships with colleagues.