It is a fast-growing problem, particularly with workers becoming increasingly concerned about their futures in the economic downturn, and costs Britain more than £30 billion every year in lost production.
The step-by-step guide shows employers and managers how to spot early the signs of mental ill-health; raise awareness of mental health issues among managers and staff; develop a culture where an employee feels comfortable disclosing their condition; approach an employee who may have a mental health condition; and try to help them cope with it or overcome it so they can work effectively again.
It also explains how to be aware of patterns of behaviour, build a rapport with an employee, talk to them in a range of scenarios, and what practical steps can be taken.
Acas chief executive John Taylor said: "The stumbling block at the moment is that many employers and managers shy away from dealing with mental illness at work because it can be hard to pin down and it is a very sensitive matter to deal with.
"People tend not to want to talk about mental illness because they think it is something disturbing that it is easier to avoid.
"But we all need to develop a new way of looking at mental health and break down the taboo. There needs to be a willingness to discuss mental health and a culture where employers understand it and try to help their employees recover from mental illness.
"Our mental well-being is as much a part of health as our physical health. And we need to take into consideration that one in four of us will suffer a mental health problem at some point in our lives."
Taylor added: "Managers are not expected to become professional counsellors. They should start by handling mental ill-health in the same way they would a physical illness by focusing on good communication, becoming aware of the issues and empathising."
The key for managers is to identify factors they can control and those they cannot.
"Managers can control workloads, work variety and relationships, communication and bullying, and strategies for the employee to cope at work, and they may be able to have some influence over financial rewards, status, sense of purpose and levels of stress.
"But there can be circumstances outside an employer's control, such as childhood experiences, family relationships, addiction problems and bereavement. However, an employer, by creating a supportive environment at work where people feel able to disclose their problems, can help them address their issues and remain productive at work.
"Also, there may be times when an employer will need to refer an employee to outside, specialist sources for help and advice."
Acas has compiled the guide with NHS agency Workways, which specialises in advising on dealing with mental ill-health at work.
Another mental health organisation, charity the Centre for Mental Health, says a total of 91 million work days are lost to mental ill-health every year. But the lost days account for only half the £30billion cost of reduced productivity, with the rest taken up by employees turning up for work when unwell and not performing at their best.
The Government's Department of Health estimates that one in four people will suffer a mental health problem, while a new government-funded study says almost one in three workers now suffer anxiety or panic attacks due to work pressures.
Another survey by the CIPD, and healthcare insurance company Simplyhealth found that stress is now the number one cause of long-term absence.
The guide comes during the Time To Change campaign run by charities Mind and Rethink to end the stigma of mental ill-health. The campaign is backed by the Government and runs until March, 2015