Born in Bedford, New Hampshire, Atwood attended the public schools as a child.

He moved to Freeport, Illinois in 1845 and engaged in agricultural pursuits before moving to Madison, Wisconsin in 1847 and for forty-two years was editor and publisher of the Wisconsin Journal.

Atwood was commissioned a major general in the Wisconsin Militia by Governor Alexander W. Randall in 1858, was a member of the Wisconsin State Assembly in 1861, was a United States assessor for four years and served as mayor of Madison, Wisconsin in 1868 and 1869.

In 1870, he was elected a Republican to the United States House of Representatives to fill a vacancy caused by the death of Benjamin F. Hopkins.

Afterwards, Atwood resumed activities in the newspaper business, was a commissioner at the Centennial Exposition representing the State of Wisconsin from 1872 to 1876 and was a delegate to the Republican National Convention in 1872 and 1876.