Dame Sally Davies said there were signs funding was being cut at a time when the cost to the economy was rising.
Her annual report said mental illness led to the loss of 70 million working days last year – up 24% since 2009.
As well as calling for greater emphasis on mental illness in the NHS, she also said employers could play a role too.
She recommended they allowed people with mental health problems the option of flexible working to keep them in employment and maintaining regular contact during sickness leave.
Overall, mental illness costs the economy between £70bn and £100bn in lost productivity, benefit payments and absence from work.
In terms of NHS spending, it accounts for 13% of the budget despite causing 28% of illness.
Dame Sally said there were signs spending in real terms had been cut since 2011 – and called for this disinvestment to stop.
On top of that, she said, access to services needed to improve as three-quarters of people with a mental illness did not receive treatment.
She suggested targets for waiting times – as have been introduced in other areas of the health service such as A&E units and for routine surgery – could make a difference.
Young people in particular needed better access to support, she said, as half of adults with mental health problems develop them before the age of 15 and three-quarters by 18.
Dame Sally said the costs were “astounding” and NHS bosses needed to treat mental health “more like physical health”.
“Anyone with mental illness deserves good quality support at the right time,” she said.
“Underinvestment in mental health services, particularly for young people, simply does not make sense economically.”
She told the BBC she was particularly concerned about those whose mental health affected their ability to work.
“One in four adults suffers some form of mental ill-health all of the time,” she said.
“But what I’m concerned about is how do we support people with mental ill-health who fall out of work.
“How do we prevent them falling out of work and how do we get them back into the workforce because this is costing us a lot in upset for those people and their families as well as our economy,”
She went on to say that employers can make a significant difference to the health of their staff.
“They can make it by actually talking about it, knowing how their people are, whether they have ill health, supporting them by giving them flexible working if they need it, by reducing stigma.”
Dame Sally also said that if an employee went sick, an employer could call them up “sensitively and talk to them, you can help them get back to work earlier”.
It comes after a number of reports have highlighted how mental health services are being squeezed.
In January Deputy Prime Minister Nick Clegg described mental health as the “poor cousin” in the health service.
Laura May, who has a borderline personality disorder, told the BBC that having an understanding employer is the most important factor for her.
“My NHS care, while it is very good, is not the reason I work well. The reason I work well is because of my employer who supports me.
“If I am particularly unwell, crisis care is very important for me because it will get me back to work very, very quickly. I think crisis care in England is not particularly good, so that is definitely an area that could be improved.”
Andy Bell, deputy chief executive of the Centre for Mental Health, said: “Better, faster and earlier help for mental health is vital to improve people’s lives and represents excellent value for money.
“Mental health and wellbeing should be a major priority in 21st Century public health.”
Care and Support Minister Norman Lamb said the CMO’s recommendations would be “considered carefully”, adding attempts were being made to make mental health more of a priority, including the possibility of introducing targets.
“I want to build a fairer society where mental health is treated with the same importance as physical health,” he said.