Professor Simon Wessely said that some youngsters with mental health issues get "literally no" treatment.
Meanwhile seven in 10 people with common mental illnesses get no treatment, he said in his first interview since taking up the post in June.
Prof Wessely told the Guardian that the nation would not tolerate it if these figures related to cancer care.
He said that in some areas patients are routinely waiting for up to two years for care.
He told the newspaper: "I'm giving a talk soon. I'm thinking of starting it: 'So, we have a problem in cancer service at the moment. Only 30% of people with cancer are getting treatment, so 70% of them don't get any treatment for their cancer at all and it's not even recognised.' And there will be a pause and I will say, 'OK - I'm not talking about cancer, but if I was, you'd be absolutely appalled and you would be screaming from the rooftops'.
"People are still routinely waiting for - well, we don't really know, but certainly more than 18 weeks, possibly up to two years, for their treatment and that is routine in some parts of the country.
"Some children aren't getting any treatment at all - literally none. That's what's happening."
He said a larger proportion of people with severe mental illness have access to treatment but the figure still only stands at around 65%, he said.
Prof Wessely said that although the NHS has an aspiration to deliver parity of esteem between physical and mental health services "the gap is now so big and yet there is no more money".
He added: "If people really want true parity in the sense of actual 90% of mental health patients are treated within 18 weeks, just like they are for other disorders, that is going to have to mean money will have to move from acute to mental health. Genuine money.
"As there is no more money, that would mean significant losses in other sectors."
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17 Facts About Depression
The news comes after the death of actor Robin Williams who was found hanged in his bedroom at his California home earlier this week.
The Mrs Doubtfire star was discovered by his personal assistant on Monday morning after he failed to answer his door.
Williams, 63, had grappled with severe depression. He had also battled addiction for decades and checked himself back into rehab last month.
But his representative said at the time that he had not fallen off the wagon and was ensuring that he focused on his commitment to stay sober.