Brown ground beef in a large skillet over medium-high heat. Drain off any excess fat.
Add onion and cook until softened, about 5 minutes.
Stir in ketchup, brown sugar, Worcestershire sauce, mustard, and garlic powder. Bring to a simmer and cook for 10-15 minutes, or until the sauce has thickened.
Serve on hamburger buns.
Notes
For a spicier Sloppy Joe, add a pinch of cayenne pepper or a dash of hot sauce.