Mathias died Monday at his home in Chevy Chase, Md., from complications of Parkinson's disease, said his sons Charles and Robert.
Mathias' career spanned the turbulent years of the Vietnam War and the civil rights movement, and he often found himself at odds with his party on those and other issues.
In retirement, Mathias broke with his party again when he endorsed Sen. Barack Obama a week before the 2008 presidential election. He said Obama was "better suited to recharge America's economic health, restore its prestige abroad and inspire anew all people who cherish freedom and equality."
Mathias was elected to the House of Representatives in 1960 from a western Maryland district and quickly gained a reputation for bucking the Republican Party. Election to the Senate followed eight years later. He was a strong supporter of civil rights legislation, favored allowing the District of Columbia to govern itself and supported a federal ban on inexpensive handguns known as Saturday night specials.
When conservative Arizona Sen. Barry Goldwater won the GOP presidential nomination in 1964, Mathias would not endorse him by name, saying only that he supported all Republican candidates.
Mathias generally supported President Nixon on economic issues, but opposed two of the president's conservative Supreme Court nominees and voted against the administration's attempts to weaken the 1965 voting rights acts.
In his home state, Mathias was considered to be the father of the massive state and federal program to clean up Chesapeake Bay.
On June 22, 1973, he began a five-day, 450-mile tour of Maryland's portion of the bay, meeting with more than 150 scientists, business leaders, conservationists, farmers and watermen. That tour laid the groundwork for the eventual partnership among Maryland, Virginia, Pennsylvania, the federal government and the District of Columbia to preserve the bay.
Mathias served three terms in the Senate before deciding not to seek reelection in 1986.
Charles McCurdy Mathias Jr., known as Mac, was born July 24, 1922, in Frederick, Md. In his 2008 essay endorsing Obama, he wrote that in 1860, "my great-grandfather ran for the Maryland Senate from Frederick on the anti-slavery Republican ticket. At the top of that ticket was Abraham Lincoln."
Mathias received a bachelor's degree from Haverford College and a law degree from the University of Maryland.
He served in World War II, enlisting as an apprentice seaman in 1942 and earning a commission as an ensign. He rose to the rank of captain in the Naval Reserve.
After serving as an assistant state attorney general and city attorney for Frederick, Mathias was elected to the Maryland House of Delegates in 1958. Just two years later, he was elected to the U.S. House of Representatives for the first of four terms.
Besides his sons, Mathias is survived by his wife, Ann Bradford Mathias, and two grandchildren.