Complex sentence: His father, George John Mitchell, Sr. (born Joseph Kilroy), was of ethnic Irish descent but was adopted by a Lebanese family when he was orphaned; he was a janitor at Colby College in Waterville, where Mitchell was raised.
Split into simpler sentences: His father, George John Mitchell, Sr. (born Joseph Kilroy), was of ethnic Irish descent but was adopted by a Lebanese family when he was orphaned. Mitchell's father was a janitor at Colby College in Waterville, where Mitchell was raised.