Hathaway was born in Massachusetts and served as a pilot during World War II. He flew dozens of missions in a B-24 bomber before being shot down over Romania shortly after the D-Day invasion. The farmer who found Hathaway turned him over to the Germans and he was held as a prisoner of war until his camp was liberated by the Russians.
He returned to the United States and eventually enrolled in Harvard Law School. After that, he served as a prosecutor in Maine before running for and winning the state's second congressional district seat in 1964. He served four terms in the House before deciding to take on Republican Sen. Margaret Chase Smith, a legend in Maine politics.
Hathaway defeated Smith and served one term in the Senate before being defeated by William S. Cohen in 1978, ending his time in Congress.
He retired from public life in 1996.
The Kennebec (Maine) Journal said colleagues remembered Hathaway as a classic retail politician who enjoyed traveling to the most rural portions of the state to meet and talk with constituents.
"Bill had a hell of a personality. I never saw anybody who liked more people," said Clarence "Chub" Clark, who served as Hathaway's representative in Aroostook County during his congressional career and had remained close friends with him ever since. "He was very honest and very hardworking."
King, who got his start in politics as a young member of Hathaway's staff, called him a "dear friend [and] cherished mentor."
"Indeed, I will always be grateful for the opportunity to work for Bill, and I will always treasure the sage wisdom and advice he provided to me throughout the years, both as governor and as senator," King said in a statement. "His example is one that continues to inspire me each and every day."