Circle masks are great for profile cutouts, spotlight reveals, or simple vignettes that need a soft, rounded edge.
Steps to create a circle mask
Select the layer you want to mask. Press and hold on the shape tools and choose the Ellipse Tool.

Hold Shift while dragging on your comp viewer to get a perfect circle. Hold Space while dragging to reposition the circle before you let go.

Press MM to refine Feather and Expansion for a soft edge or tighter fit.

One more thing
If you need a mask perfectly centered in your composition, first select the layer, then double-click the Ellipse Tool. After Effects automatically creates a centered mask that matches the layer size — way faster than dragging it manually.
Related how-tos
How to use the pen tool in After Effects
How to feather a mask in After Effects
How to animate a mask path in After Effects
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