Born on May 21, 1847 at Rockland, Maine, William Dwight Pennell was educated in the public schools of that city.
At the age of sixteen he moved to Lewiston with his family and from then on became a Horatio Alger story in the mills of Lewiston.
He began work in the Porter Mill in 1863 as a bobbin boy and then moved on to the Franklin Company as a draftsman. After three years with the Franklin Company he became paymaster of the Lincoln Mill and in November of 1872 he became superintendent of the Lincoln Mill.
By 1890 he had become sales agent for the Hill Manufacturing Company and attained a position of prominence in the manufacturing work of Lewiston.
From 1870 onward he held numerous public offices in the city of Lewiston, including City Councilor and President of the Board of Aldermen.
In 1881 he was elected to fill the vacancy in the Legislature caused by the death of I.N. Parker and from 1883 to 1886 he served in the Maine Senate, for the latter two years as its President.
Returning to private life, Pennell held numerous positions in various charitable and philanthropic organizations including the presidency of the Central Maine Hospital and the Lewiston Public Library.
He was director of the Maine Automobile Association and a member of the Executive Committee of the State-Wide Good Roads Organization.