Her death was first reported by the Telegraph newspaper in Britain. The cause was not disclosed.
Mrs. Fawcett was still in her teens when she received a letter from a friend in February 1940, in the early months of the war.
"I'm at Bletchley and it's perfectly frightful," her friend wrote. "We're so overworked, so desperately busy. You must come and join us."
Fluent in German and driven by curiosity, Mrs. Fawcett - then known by her maiden name, Jane Hughes - found work at Britain's top-secret code-breaking facility at Bletchley Park, about 50 miles northwest of London. Of the 12,000 people who worked there, about 8,000 were women.
Bletchley Park later became renowned as the place where mathematician Alan Turing and others solved the puzzle of the German military's "Enigma machine," as depicted in the 2014 film The Imitation Game.
Turing worked in Hut 8 at Bletchley Park, while Mrs. Fawcett was assigned to Hut 6. She was part of an all-female team whose job was to monitor messages from the German army and air force. Conditions in the single-story wooden buildings were hardly ideal.
"It was just horrid - there were very leaky windows," Mrs. Fawcett recalled in a 2015 interview with the Telegraph. "So it was very cold, with just a frightful old stove in the middle of the room that let out lots of fumes but not much heat, and just one electric bulb hanging on a string, which was quite inadequate. We were always working against time, there was always a crisis, a lot of stress, and a lot of excitement."
In May 1941, the British navy was searching for Germany's most formidable battleship, the Bismarck, which had last been seen near Norway. Mrs. Fawcett was transcribing an intercepted message from the headquarters of the Luftwaffe, the German air force, when she noticed a reference to the French city of Brest.
In a reply to a Luftwaffe general whose son was aboard the Bismarck, a German officer noted that the battleship was headed to Brest for repairs.
Mrs. Fawcett relayed her discovery to her supervisors, and within a day the Bismarck was spotted by reconnaissance aircraft in the Atlantic Ocean, about 700 miles off the coast of Brittany. British warplanes and naval vessels descended on the Bismarck, which was sunk on May 27, 1941. More than 2,000 German sailors were killed.
The sinking of the Bismarck marked the first time that British codebreakers had decrypted a message that led directly to a victory in battle. Cheers erupted among the staff at Bletchley Park, but their celebration remained private.
Mrs. Fawcett's work was not made public for decades. Along with everyone else at Bletchley Park, she agreed to comply with Britain's Official Secrets Act, which imposed a lifetime prohibition on revealing any code-breaking activities. It wasn't until the late 1990s that her role in the sinking of the Bismarck began to come to light.
"My husband had been in the navy and done all these heroic things in every quarter, so of course we all talked about him and those brilliant young adventurers who saved Britain - well, saved the world," Mrs. Fawcett said last year.
"So when everything we had done, which we knew had been very hard work and incredibly demanding, suddenly showed its head and we were being asked to talk about it, it felt quite overwhelming. I'd never told a soul, not even my husband. My grandchildren were very surprised."
Janet Caroline Hughes was born March 4, 1921, and grew up in London. Her father was a lawyer.
Mrs. Fawcett, who dropped the final letter in her first name, was a promising ballet dancer until she grew too tall. She then studied German in Switzerland before returning to England.
After working at Bletchley Park for five years, Mrs. Fawcett attended the Royal Academy of Music and had a 15-year career as an opera singer and recital soloist. In the 1960s, she began working at the Victorian Society, an organization devoted to preserving architecture from that era.
She was a passionate champion of the sturdy and ornate 19th-century buildings, with a particular interest in train stations slated for demolition by the British rail service. Railway officials dubbed her "the furious Mrs. Fawcett."