Updated January 8, 2019
If you are earning more than $1,220 (as of 2019) per month, then yes, you'll have to quit your job, or work fewer hours, to be considered for disability benefits. Social Security considers anything above that amount to be substantial gainful activity, or SGA. The SGA amount is adjusted each year according to the current price/wage index.
If you earn more than the SGA, you will not receive any Social Security disability benefits under the SSDI or SSI program (unless you are legally blind and applying only for SSI). This is true even if if you are still working your job despite the fact that you are suffering incredible pain or fatigue at work. Nor does it matter if you have a documented medical condition that has caused you to earn substantially less each month or caused you to go from full to part-time work.
However, if you require numerous accommodations to help you do your work (such as assistive devices or help from other workers), this may affect how Social Security counts your earnings, meaning that your income may fit under the SGA amount even if the monthly dollar amount is more than $1,220. For more information, see our article on part-time work and disability.
If you worked at a job for less than six months and had to quit because of your medical condition, Social Security might consider this an "unsuccessful work attempt" (UWA) rather than substantial gainful activity that would keep you from getting benefits. A short period of work has to meet certain criteria to be considered a UWA. For more information, see our article on what Social Security considers an unsuccessful work attempt.
If you are a small business owner or otherwise self-employed, Social Security uses different rules to decide if you are doing substantial gainful activity. For more information, see our article on SGA.
Social Security will generally not deny you because you are working if you are making less than the SGA amount. However, if you are able to work a significant amount of time, even if you are earning less than the SGA amount, Social Security will take this into account further on in the evaluation process when deciding if you are disabled. For instance, you could work 25 hours per week making $9 per hour and your monthly income would be $967, under the SGA limit. But when Social Security sees that you are able to work 25 hours per week, the agency will take this into account to determine if you are disabled (unless you have a medical condition that automatically qualifies for disability benefits, such as statutory blindness, esophageal cancer, ALS, or an organ transplant).
You don't have to have quit work for 12 months before you can file for disability, as some people erroneously believe. Your disability has to either be expected to last 12 months or to have already lasted 12 months. However, some disability lawyers do advocate waiting to apply for benefits for at least a few months after quitting. For more information, see our article on when to apply for Social Security disability.