What is mcg to mL?

If you mean micrograms when you write mcg, that is a mass measurement and ml (milliliters) is a volume measurement so strictly speaking, there is no conversion...

But

Many things have a density of about 1. A milliliter of water (density 1) has a mass of exactly one gram.

A microgram is one millionth of a gram.

micrograms / 1 000 000 = grams (ml of water etc)