I have a mental disability. It affected my career in the past but since getting diagnosed and treated for it a few years ago my career has improved significantly and I have been able to put in consistent work.

My disability is not obvious or visible. Up until now I have only told close family.

Recently, I have been considering job hunting to try to improve my salary. I currently make around 50k USD. I'm based in Japan so this salary is not terribly low but also not that high.

In Japan, companies have a requirement to hire a certain percentage of disabled people. Looking at the Google recruitment site I can see they have a program (gReach) specifically for registered disabled people.

Is it a good idea to disclose my disability during the application process? On one hand I feel like it might encourage companies to interview/hire me to fill their quota. On the other hand I worry that companies may be prejudiced against disabled people and use it to disqualify my application.

If their application process has a place to disclose it, do so there. If not, do not volunteer the info. The process-driven disclosure should keep the information separate from hiring decisions, which is what you want as it supports their hiring needs without attaching quotas to you personally. After all, quotas can cut both directions.

Once you are hired, disclose it on the very first day, with a letter from a doctor that details what accommodations are required for you to do the job. Unless you don't need accommodations, in which case do not disclose it.

Are you officially registered as disabled (or planning to be)? Admittedly I haven't looked into deeply, but I got the impression that that system is mostly for people who require some form of government services for their disability, which may not cover you if you're not physically disabled and are currently coping with daily work/life. Also, I don't know your visa situation, but I would be concerned that registering as potentially in need of government assistance would be counted against me when renewing or applying for PR.

Other than that, my personal policy is to not disclose unless I need some kind of accommodation (which I generally don't). I think the risks of having it consciously or subconsciously counted against me outweigh the potential benefits of being favored by a quota.

Unless the company you're applying to has a strong proven track record for helping those with disabilities I would suggest to not disclose it. It only takes one person in your management chain to discriminate against you.

You indicate you are in Japan so I think the first step is to find out what the legal definition of what "disabled" means there. Maybe find an employment attorney or equivalent to find out what is covered and what provisions you may be able to ask for, what hiring practices are illegal, etc. It will vary per country so unless someone on here is also in Japan and has a similar diagnosis I would caution any specific advice.

I think it depends on the company you're applying for:

Disclose it to companies who have programs to support people with disabilities

Don't disclose it to companies who don't have a support program

Maybe. The 7-8 workplaces I've been to have been remarkably homogeneous with very few people sticking out in any way. All-male teams or teams where most people were the same age. If one goes by what they do rather than what they say then being average is a very desirable trait. And I think people hide their peculiarities.

Are mental health issues (e.g. depression or anxiety) considered "mental disability"?

I have MS which causes urinary incontinence and my mobility is impaired (unable to stand for too long, cannot walk for longer than 100-200 meters at a time and so on).

Be honest, but use discretion in what you discuss. If your disability will directly impact job performance, say so. Otherwise, focus on experience and results.

Disability hiring quotas are most effective for those with mobility impairments.

In the U.S., medical information disclosed to insurers has strong privacy protection under HIPAA. The HR department must keep information about a treated disability confidential, even if you haven't discussed it with coworkers.

Your improved condition after diagnosis and treatment is a powerful affirmation of your character and determination, qualities that would benefit a new team.