The Healthcare Commission review of 174 mental health teams in England found gaps in out-of-hours care, talking therapies and access to information.
The watchdog rated one in 10 as excellent, with nearly half just getting a fair grade.
Campaigners said the findings were concerning, and NHS bosses said there was room for improvement.
The vast majority of people with mental health problems are treated out of hospital by community mental health specialists, including GPs, nurses, counsellors and social workers.
The Healthcare Commission, which analysed performance data and patient surveys, found while the majority of local mental health teams had out-of-hours services in their plan, just 49% of people with problems had the phone number of someone they could contact after office hours.
And 59% of the partnerships, known as local implementation teams (LITs), scored poorly when it came to providing access to crisis accommodation out-of-hours.
The Healthcare Commission rated 9% of LITs as excellent, 45% as good, 43% as fair and 3% as weak.
It also found a greater need for access to talking therapies - such as counselling, cognitive behavioural therapy or psychoanalysis.
National guidelines say all people with schizophrenia or suspected schizophrenia should be offered appropriate talking therapies.
The review found that only half of people questioned had such access and in 20% of LIT areas the figure was "significantly lower".
It also said the management of medicines for patients with schizophrenia needed to improve - with 84% of LITs deemed fair or weak.
The watchdog said 89% of services had not adequately recorded side effects or how well patients had responded to drugs.
Healthcare Commission chief executive Anna Walker said: "The majority of people who suffer from mental illness receive their treatment in their own community, not in hospital.
"They want to remain in the community and this helps them get better. But for care in the community to work for the mentally ill, more access is needed to talking therapies and out-of-hours crisis care.
"Mental health crises don't keep office hours and the service must be flexible enough to tackle this."
Professor Louis Appleby, the national director of mental health, said there were about 1,700 more clinical psychologists, and nearly 1,000 more primary care therapists working in the NHS in recent years.
"But in a way these new therapies - cognitive therapy is the main one - are a victim of their success.
"There's growing research evidence that they can be used for a whole range of conditions, so of course the demand and the need is much greater, and it's far outstripping what we can provide at the moment."
Room for improvement
Nigel Edwards, of the NHS Confederation, which represents health trusts, said: "Mental health has often been a Cinderella service in the NHS, yet one in four of us will experience a mental health problem every year.
"So it is reassuring that the review has praised local implementation teams for generally performing well.
"Of course, there is always room for improvement and the report presents some serious challenges for mental health trusts and their partners in the services that they provide."
Mental health charities have expressed their concern at the commission's findings.
Paul Farmer, chief executive of Mind, said: "We're concerned about the large number of people who don't have access to basic treatments, like cognitive behavioural therapy, which is simply not available in many areas of the country."
Sane's chief executive Marjorie Wallace said: "This report shows that the community care policy still fails thousands of mentally ill people and their families.
"It is disturbing that this strong indictment of out-of-hours community care should come at the very time that mental health budgets are being slashed."