It contains the communities of Acmetonia, Harmarville, Harmar Heights, and Chapel Downs.

Deer Creek flows through the township; it joins the Allegheny at Harmar.

Bordering the Allegheny River, Harmar runs adjacent with Oakmont to the south (with a direct link via Hulton Bridge) and Plum to the south and southeast.

At the 2000 census there were 3,242 people, 1,522 households, and 882 families living in the township.

There were 1,637 housing units at an average density of 274.3 per square mile (105.9/km2).