Kavanaugh was born in Newcastle, Maine. His childhood home was a haven for missionary priests, including Fr. Cheverus. After study at Georgetown College, St. Mary's, Baltimore, and in Europe, he was elected to state offices on Maine, and then to U.S. Congress for two terms beginning in 1831. After a term as a diplomat in Portugal, he returned to state politics, serving in the senate and briefly as governor. He also participated in negotiations in Washington to settle the Maine-Canada border, which resulted in the Webster-Ashburton Treaty of 1842.