I'm an adult but I lived at home with my mom in 2018. Only income was a withdrawal from my 401K. If my mom claims me as a dependent how will that impact my taxes?
Since in 2018 personal exemptions are removed you get no benefit on your return from claiming yourself. Your mom could get a credit for a qualifying dependent ($500 nonrefundable). Now some other credits and deductions require you to claim yourself, but from your explanation it does not sound like you would need to worry about those.
When you say "adult", I am assuming that you are over age 23; and you are not married; and you lived with your mother for over half of the year; and your mother provided over half of your support; and your mother is not a dependent of someone else. I am also assuming that the money withdrawn from your 401k is not from a Roth 401k and is therefore fully taxable as ordinary income. So the answer here depends upon the amount of money that you withdrew. In order to claim you as a "qualifying relative" for tax-year 2018, then your total income must be under $4,150. (Your 401k withdrawal adds into the total of any of your other income such as Social Security (even if tax exempt) and W-2, self employed, or interest income, etc.) Since you say that all other income is zero, then if your 401k withdrawal is under $4,150, then you appear to be her "qualifying relative". This would qualify your mother to file with Head-of-Household filing status and would provide her with a $500 credit for "other dependents". You would file your own return to get any withholdings returned to you or to pay any penalties (such as possible early withdrawal under age 59-1/2). In that case, you would need to be careful to click the questions, as you complete your tax return, such that you state you are a dependent of someone else and you do not take your own dependency exemption. On the other hand, if you withdrew $4,150 or more from the 401k, then you are not a "qualifying relative", and your mother cannot claim you as a dependent. In that case, you would simply file your return in your usual manner.