If you have ArcGIS Desktop, you can follow the directions below and it will take about 5 minutes. Download US Counties from ESRI; Follow ESRI's instruction at HowTo: Find the centroid of polygons using Calculate Geometry; This results in a table of Latitude and Longitude attributes as well as points for the centers on your map.

Map Viewer displays geographic coordinates in several common formats. You can change the location units used to display the coordinates, capture a point to see its exact coordinates, input coordinates and go to that location on the map, and format how each location unit displays.

As a new user to ArcGIS Online, I am trying to create a map using ArcGIS Online that has a layer to show the boundaries of about 20 counties in the United States (and include Washington DC). So, first I added the ESRI "USA Counties Generalized Boundaries" from Living Atlas.

This layer provides generalized county (or county equivalent) boundaries for the United States in the 50 states and the District of Columbia, developed by Esri from U.S. Census Bureau sources and updated as boundaries change.

USA Counties (Generalized) provides the county boundaries of the United States in the 50 states and the District of Columbia.