They point to figures showing a nearly five-fold increase over two years in the number of children and young people waiting more than 14 weeks to be seen.
Leader Kirsty Williams said mental health was seen as an "afterthought".
The Welsh government said it would spend an extra £7.6m a year to ensure young people were assessed quicker.
The Liberal Democrats found that in April 2015 there were 1,332 children who had been waiting over 14 weeks to be assessed by psychiatric services, up from 282 in April 2013.
"These figures are appalling and show that mental health waiting lists are spiralling out of control," said Ms Williams.
"For too long mental health services have been an afterthought of governments. This complacency means our mental health services are miles behind where they should be."
A Welsh government spokesman said an extra £7.6m a year would be spent to ensure young people receive "the right treatment at the right time".
He added that work was needed to cut the number of "unnecessary referrals", with a 100% increase in children and young people seen by child and adolescent mental health services (CAMHS) over four years.
"Services tell us that around a third do not have a problem requiring any intervention and are discharged at once, and a further third of referrals do not need specialist treatment," he said.

The Scottish, Welsh and Northern Irish administrations have their spending level dictated by the Barnett formula.
Under it, extra funding or cuts for England has an affect on the other nations, dependent on their population size and the powers devolved to them.
Stormont's finance committee says parts of the formula could be more open.
That could include requiring the Treasury to publish data upon which Barnett-related decisions are based.
As an example, the committee pointed to the initial Treasury decision to classify infrastructure spending on the London Olympics as having UK-wide rather than England-only benefits.
That meant there were was no so-called consequential spending for the devolved governments.
The devolved nations did eventually get extra spending, but only about Â£30m rather than the Â£330m they thought they were due.
Some expert witnesses suggested that an independent body, such as the Office for Budget Responsibility, should be given an oversight role.
Prof Gerry Holtham, who produced a report on funding and financing in Wales, told the committee that Northern Ireland would benefit from the formula "being put on a proper statutory basis with an element of independence from Treasury introduced".
The committee also said that the executive should take "pre-emptive measures" for the event that the government decides to dispense with or alter the Barnett formula.
It said this could include piloting a new needs-based formula and developing a "fully considered position" on what arrangements will be in Northern Ireland's best interests.
Mental health waiting lists are "spiralling out of control", the Welsh Liberal Democrats have claimed, accusing ministers of "complacency".
[NEXT_CONCEPT]
The finance ministers of the devolved UK administrations should present joint proposals for improving the way they are funded, a Stormont committee says.