According to Walter Lord, "After [the Titanic] sank, the New York American broke the news on April 16 with a lead devoted almost entirely to John Jacob Astor; at the end it mentioned that 1800 others were also lost." Astor's prominence led to the creation of many exaggerated and unsubstantiated accounts about his actions during the sinking of the Titanic. One story alleges that he opened the ship's kennel and released the dogs, including his own beloved Airedale, Kitty; in another, he placed a woman's hat on a boy to make sure the child was able to get into a lifeboat. Another legend claims that after the ship hit the iceberg, he quipped, "I asked for ice, but this is ridiculous." These stories appeared in newspapers, magazines, and even books about the sinking. In reality, none of the claims about Astor's actions were substantiated, as nobody who recognized him survived other than the women who boarded lifeboats relatively early on. Wade wrote that the ice joke is almost certainly apocryphal, as Astor was not known for making jokes, and that the story about the hat (like many other "survivor stories" published shortly after the sinking) may have been invented by the reporter. Another legend is that Astor was crushed to death by one of the ship's falling funnels. However, this legend was not true.

John Jacob Astor IV was born on July 13, 1864 at his parents' country estate Ferncliff in Rhinebeck, New York. He was the youngest of five children and only son of businessman, collector, and race horse breeder/owner william Backhouse Astor, Jr. and socialite Caroline Webster "Lina" Schermerhorn. His four elder sisters were Emily (1854–1881), Helen (1855–1893), Charlotte (1858–1920), and Caroline ("Carrie") (1861–1948). He was a great-grandson of fur-trader John Jacob Astor and Sarah Cox Todd (1761–1834), whose fortune made the Astor family one of the wealthiest in the United States. Astor’s paternal grandfather william Backhouse Astor, Sr. was a prominent real estate businessman. Through his paternal grandmother Margaret Alida Rebecca Armstrong (1800–1872), Astor was also a great-grandson of Senator John Armstrong, Jr. and Alida Livingston (1761–1822) of the Livingston family. His maternal grandparents were Abraham Schermerhorn, a wealthy merchant, and socialite Helen Van Courtlandt White. He was also a nephew of financer/philanthropist John Jacob Astor III and grandnephew of occasional poet John Jacob Astor, Jr. (1791–1869). His sister Helen's husband was diplomat James Roosevelt "Rosey" Roosevelt, half-brother of President Franklin Delano Roosevelt of the Roosevelt family. Another sister, Carrie, a noted philanthropist, was the wife of Marshall Orme Wilson (1860–1926), brother of banker Richard Thornton Wilson, Jr. and socialite Grace Graham Wilson. Astor was also a first cousin of william Waldorf Astor, 1st Viscount Astor.

On February 17, 1891, Astor married socialite Ava Lowle Willing, a daughter of Edward Shippen Willing and Alice Barton. The couple had two children:

From 1894 to 1896, he was a colonel on the military staff of New York Governor Levi P. Morton. Shortly after the outbreak of the Spanish–American War in 1898, Astor personally financed a volunteer artillery unit known as the "Astor Battery", which saw Service in the Philippines. In May 1898, Astor was appointed a lieutenant colonel in the U.S. Volunteers and served as an officer on the staff of Major General william Shafter in Cuba, during the Santiago Campaign. He was later given a brevet (honorary promotion) to colonel in recognition of his services. He was mustered out of the Volunteer Army in November 1898.

During the war, he allowed his yacht, the Nourmahal, to be used by the U.S. government. He appeared in the films President McKinley's Inspection of Camp Wikoff (1898) and Col. John Jacob Astor, Staff and Veterans of the Spanish–American War (1899). As a result of his military Service, Astor was entitled to the Spanish Campaign Medal. After the war, Astor was often referred to as "Colonel Astor".

Astor and Willing divorced in November 1909. Compounding the scandal of their divorce was Astor's announcement that he would remarry. At the age of 47, Astor married 18-year-old socialite Madeleine Talmage Force, the sister of real estate businesswoman and socialite Katherine Emmons Force. Their parents were william Hurlbut Force and Katherine Arvilla Talmage. Astor and Force were married in his mother's ballroom at Beechwood, the family's Newport, Rhode Island, mansion. There was also much controversy over their 29-year age difference. His son Vincent despised Force, yet he served as best man at his father’s wedding. The couple took an extended honeymoon in Europe and Egypt to wait for the gossip to calm down. Among the few Americans who did not spurn him at this time was Margaret Brown, later fictionalized as The Unsinkable Molly Brown. She accompanied the Astors to Egypt and France. After receiving a call to return to the United States, Brown accompanied the couple back home aboard the RMS Titanic.

Astor died in the sinking of the RMS Titanic during the early hours of April 15, 1912. He was among the 1,514 people on board who did not survive. Astor was the richest Passenger aboard the RMS Titanic and was thought to be among the richest people in the world at that time, with a net worth of nearly $87 million when he died (equivalent to $2.21 billion in 2017).

Astor's fame has made him a frequent character in films about the Titanic. German actor Karl Schönböck played Astor in the 1943 Nazi propaganda film Titanic. william Johnstone played Astor in the 1953 film Titanic, and in the 1997 version of Titanic he was played by Eric Braeden. In the 1996 miniseries, he was played by Scott Hylands. Astor was also portrayed by David Janssen in the 1979 film S.O.S. Titanic. Perennially, in the town of Astoria, Oregon (so named for Astor's patrilineal great-grandfather), he is portrayed by a local amateur actor in street corner vignettes. He was played by Miles Richardson in the 2012 Titanic miniseries. In April 2012, Astor was portrayed by his great-grandson Gregory Todd Astor in "Titanic the Musical".

Astor's estate "Ferncliff", north of the town center of Rhinebeck, New York, with a mile and a half of Hudson River frontage in the picturesque Lower Hudson River Valley, had been purchased piecemeal by his Father in the mid-19th century; Astor was born there. His father's Italianate house of 1864 was partly rebuilt in 1904 to designs by Stanford White of McKim, Mead, and White, retaining its conservative exterior, and a Sports pavilion in Louis XVI style was added. The "Casino" or "Astor Courts" reportedly housed the first residential indoor swimming pool in the U.S., an indoor tennis court with vaulting of Guastavino tile, and guest bedrooms; in the lower level were a bowling alley and a shooting range. The estate, reduced to 50 acres (200,000 m) and renamed "Astor Courts", eventually became a wedding venue. Chelsea Clinton was wed there on July 31, 2010.

When Second Officer Charles Lightoller later arrived on A Deck to finish loading Lifeboat 4, Astor helped his wife, with her maid and nurse, into it. He then asked if he might join his wife because she was in "a delicate condition"; however, Lightoller told him men were not to be allowed to board until all the women and children had been loaded. According to Titanic Passenger Archibald Gracie IV, "She was lifted up through the window, and her husband helped her on the other side, and when she got in, her husband was on one side of this window and I was on the other side, at the next window. I heard Mr Astor ask the second officer whether he would not be allowed to go aboard this boat to protect his wife. He said, "No, sir, no man is allowed on this boat or any of the boats until the ladies are off." Mr Astor then said, "Well, tell me what is the number of this boat so I may find her afterwards," or words to that effect. The answer came back, "Number 4." According to child survivor Betty, as quoted on Children on the Titanic (2014), Astor was boarding the final lifeboat with his pregnant wife when he saw two scared children on deck and stepped aside, giving his place to them. A conflicting news article posted in the Chicago Record Herald tells of Astor placing his wife into the final lifeboat then ordering Ida Sophia Hippach and her 17-year-old daughter Jean Gertrude to take the final two places before the boat set sail.

Astor left $69 million of his $85 million estate (equivalent to approximately $1.75 billion in 2017 dollars) to Vincent. This value included his estate in Rhinebeck and his yacht, the Noma. To Madeleine Force Astor, he left $100,000 as an outright bequest as well as a $5 million trust fund from which she was provided an income. Additionally she was given the use of his New York City mansion at 65th Street & Fifth Avenue and all its furnishings, his Newport mansion Beechwood and all its furnishings, pick of whichever luxury limousine she wanted from his collection, and five of his prized horses—as long as she did not remarry. His daughter Ava (who lived with her mother, also named Ava) received a $10 million trust fund. Upon turning 21, John Jacob VI inherited the $3 million trust fund Astor had set aside for him.

While traveling, Madeleine Force Astor became pregnant. Wanting the child born in the U.S., the Astors boarded the RMS Titanic on her maiden voyage to New York. They embarked in Cherbourg, France, in first class and were the wealthiest passengers aboard. Accompanying the Astors were Astor's valet, Victor Robbins; Force’s maid, Rosalie Bidois; and her nurse, Caroline Louise Endres. They also took their pet Airedale, Kitty. The Astors were deeply fond of their dog and had come close to losing her on a previous trip when she went missing in Egypt. Kitty did not survive the sinking. A short while after the Titanic hit the iceberg that caused her to sink, Astor informed his wife of the collision but told her the damage did not appear to be serious. Some time later, as the ship's lifeboats for first class were being manned, Astor remained unperturbed; he and his family played with the mechanical horses in the gymnasium. At some point Astor is thought to have sliced the Li Ning of an extra lifebelt with a pen knife to show his wife its contents, either to prove they were not of use or to reassure her that they were. He even declared: "We are safer here than in that little boat."