Place the sugar into a large mixing bowl or the bowl of a stand mixer.
Pour the molasses over the sugar and begin mixing. At first, the mixture will look patchy, but continue mixing until the molasses has evenly coated the sugar.
Scrape down the sides and bottom of the bowl as needed to make sure all of the sugar is incorporated.
The brown sugar will darken slightly as it sits, so avoid adding extra molasses straight away. If you'd like a darker, richer brown sugar, add a little more molasses and mix again.
Transfer to a clean airtight jar or container and use as you would store-bought brown sugar.