Barnum had several businesses over the years, including a general store, a book auctioning trade, real estate speculation, and a statewide lottery network. He started a weekly newspaper in 1829 called The Herald of Freedom in Danbury, Connecticut. His editorials against the elders of local churches led to libel suits and a prosecution which resulted in imprisonment for two months, but he became a champion of the liberal movement upon his release. He sold his store in 1834.

On November 8, 1829, Barnum married Charity Hallett, and they had four children: Caroline Cornelia (1830–1911), Helen Maria (1840–1920), Frances Irena (1842–1844), and Pauline Taylor (1846–1877). His wife died on November 19, 1873, and he married Nancy Fish the following year, the daughter of his close friend John Fish who was 40 years younger than him.

He began his career as a showman in 1835 when he was 25 with the purchase and exhibition of a blind and almost completely paralyzed slave woman named Joice Heth, whom an acquaintance was trumpeting around Philadelphia as George Washington’s former nurse and 161 years old. Slavery was already outlawed in New York, but he exploited a loophole which allowed him to lease her for a year for $1,000, borrowing $500 to complete the sale. Heth died in February 1836, at no more than 80 years old. Barnum had worked her for 10 to 12 hours a day, and he hosted a live autopsy of her body in a New York saloon where spectators paid 50 cents to see the dead woman cut up, as he revealed that she was likely half her purported age.

Barnum had a year of mixed success with his first variety troupe called “Barnum’s Grand Scientific and Musical Theater”, followed by the Panic of 1837 and three years of difficult circumstances. He purchased Scudder’s American Museum in 1841, located at Broadway and Ann Street, New York City. He improved the attraction, upgrading the building and adding exhibits, then renamed it “Barnum’s American Museum”; it became a popular showplace. He added a lighthouse lamp which attracted attention up and down Broadway and flags along the roof’s edge that attracted attention in daytime, while giant paintings of animals between the upper windows drew attention from pedestrians. The roof was transformed to a strolling garden with a view of the city, where he launched hot-air balloon rides daily. A changing series of live acts and curiosities were added to the exhibits of stuffed animals, including albinos, giants, little people, jugglers, magicians, exotic women, detailed models of cities and famous battles, and a menagerie of animals.

In 1842, Barnum introduced his first major hoax: a creature with the body of a monkey and the tail of a fish known as the “Feejee” mermaid. He leased it from fellow museum owner Moses Kimball of Boston who became his friend, confidant, and collaborator. Barnum justified his hoaxes by saying that they were advertisements to draw attention to the museum. “I don’t believe in duping the public”, he said, “but I believe in first attracting and then pleasing them.”

In 1843, Barnum hired the Native American dancer fu-Hum-Me, the first of many First Nations people whom he presented. During 1844–45, he toured with General Tom Thumb in Europe and met Queen Victoria, who was amused but saddened by the little man, and the event was a publicity coup. It opened the door to visits from royalty throughout Europe, including the Tsar of Russia, and enabled Barnum to acquire dozens of new attractions, including automatons and other mechanical marvels. During this time, he went on a spending spree and bought other museums, including artist Rembrandt Peale’s Museum in Philadelphia, the nation’s first major museum. By late 1846, Barnum’s Museum was drawing 400,000 visitors a year.

Barnum built four mansions in Bridgeport, Connecticut: Iranistan, Lindencroft, Waldemere, and Marina. Iranistan was the most notable, a Moorish Revival architecture designed by Leopold Eidlitz with domes, spires, and lacy fretwork inspired by the Royal Pavilion in Brighton, England. It was built in 1848 but then it burned down in 1857. The Marina Mansion was demolished by the University of Bridgeport in 1964 in order to build their cafeteria.

Lind demanded the fee in advance and Barnum agreed; this permitted her to raise a fund for charities, principally endowing schools for poor children in Sweden. Barnum borrowed heavily on his mansion and his museum to raise the money to pay Lind but he was still short of funds; so he persuaded a Philadelphia minister that Lind would be a good influence on American morals, and the minister lent him the final $5,000. The contract also gave Lind the option of withdrawing from the tour after 60 or 100 performances, paying Barnum $25,000 if she did so. Lind and her small company sailed to America in September 1850, but she was a celebrity even before she arrived because of Barnum’s months of preparations; close to 40,000 people greeted her at the docks and another 20,000 at her hotel. The press was also in attendance, and “Jenny Lind items” were available to buy. When she realized how much money Barnum stood to make from the tour, she insisted on a new agreement which he signed on September 3, 1850. This gave her the original fee plus the remainder of each concert’s profits after Barnum’s $5,500 management fee. She was determined to accumulate as much money as possible for her charities.

The tour began with a concert at Castle Garden on September 11, 1850, and it was a major success, recouping Barnum four times his investment. Washington Irving proclaimed, “She is enough to counterbalance, of herself, all the evil that the world is threatened with by the great convention of women. So God save Jenny Lind!” Tickets for some of her concerts were in such demand that Barnum sold them by auction, and public enthusiasm was so strong that the press coined the term “Lind mania”. The blatant commercialism of Barnum’s ticket auctions distressed Lind, and she persuaded him to make a substantial number of tickets available at reduced prices.

He organized flower shows, beauty contests, dog shows, and poultry contests, but the most popular were baby contests such as the fattest baby or the handsomest twins. In 1853, he started the pictorial weekly newspaper Illustrated News; he completed his autobiography a year later which sold more than a million copies over the course of numerous revisions. Mark Twain loved the book, but the British Examiner thought it “trashy” and “offensive” and wrote that it inspired “nothing but sensations of disgust” and “sincere pity for the wretched man who compiled it”.

Barnum went on to create America’s first aquarium and to expand the wax figure department of his museum. His “Seven Grand Salons” demonstrated the Seven Wonders of the World. The collections expanded to four buildings, and he published a “Guide Book to the Museum” which claimed 850,000 “curiosities”. Late in 1860, Siamese Twins Chang and Eng came out of retirement because they needed more money to send their numerous children to college. They had a touring career on their own and went to live on a North Carolina plantation with their families and slaves under the name of Bunker. They also appeared at Barnum’s Museum for six weeks. Also in 1860, Barnum introduced “man-monkey” William Henry Johnson, a microcephalic black little person who spoke a mysterious language created by Barnum. In 1862, he discovered giantess Anna Swan and Commodore Nutt, a new Tom Thumb with whom Barnum visited President Abraham Lincoln at the White House. During the Civil War, his museum drew large audiences seeking diversion from the conflict. He added pro-Unionist exhibits, lectures, and dramas, and he demonstrated commitment to the cause. He hired Pauline Cushman in 1864, an actress who had served as a spy for the Union, to lecture about her “thrilling adventures” behind Confederate lines. Barnum’s Unionist sympathies incited a Confederate sympathizer to start a fire in 1864. Barnum’s American Museum burned to the ground on July 13, 1865 from a fire of unknown origin. Barnum re-established it at another location in New York City, but this also was destroyed by fire in March 1868. The loss was too great the second time, and Barnum retired from the museum business.

Barnum claimed that “politics were always distasteful to me”, yet he was elected to the Connecticut legislature in 1865 as Republican representative for Fairfield and served four terms. He hired spies to get insider information on the New York and New Haven Railroad lines and exposed a secret that would raise fares by 20 percent. He said during the ratification of the Thirteenth Amendment to the United States Constitution: “A human soul, ‘that God has created and Christ died for,’ is not to be trifled with. It may tenant the body of a Chinaman, a Turk, an Arab or a Hottentot—it is still an immortal spirit.” He also acknowledged that he had owned slaves when he lived in the South. “I whipped my slaves. I ought to have been whipped a thousand times for this myself. But then I was a Democrat—one of those nondescript Democrats, who are Northern men with Southern principles”.

Barnum did not enter the circus business until he was 60 years old. He established “P. T. Barnum’s Grand Traveling Museum, Menagerie, Caravan & Hippodrome” in Delavan, Wisconsin, in 1870 with William Cameron Coup; it was a traveling circus, menagerie, and museum of “freaks”. It went through various names: “P. T. Barnum’s Travelling World’s Fair, Great Roman Hippodrome and Greatest Show on Earth”, and “P. T. Barnum’s Greatest Show on Earth, And The Great London Circus, Sanger’s Royal British Menagerie and The Grand International Allied Shows United” after an 1881 merger with James Bailey and James L. Hutchinson, soon shortened to “Barnum & Bailey’s”. This entertainment phenomenon was the first circus to display three rings. The show’s first primary attraction was Jumbo, an African elephant that Barnum purchased in 1882 from the London Zoo. The Barnum and Bailey Circus still contained acts similar to his Traveling Menagerie, including acrobats, freak shows, and General Tom Thumb. Barnum persisted in growing the circus in spite of more fires, train disasters, and other setbacks, and he was aided by circus professionals who ran the daily operations. He and Bailey split up in 1885, but they came back together in 1888 with the “Barnum & Bailey Greatest Show On Earth”, later “Barnum & Bailey Circus” which toured the world.

Barnum was elected for the next four Congresses and succeeded Senator Orris S. Ferry. He was the legislative sponsor of a law enacted by the Connecticut General Assembly in 1879 which prohibited the use of “any drug, medicinal article or instrument for the purpose of preventing conception”, and also made it a crime to act as an accessory to the use of contraception; this law remained in effect in Connecticut until it was overturned in 1965 by the U.S. Supreme Court in Griswold v. Connecticut. He ran for Congress in 1867 and lost to his third cousin William Henry Barnum. In 1875, he worked as mayor of Bridgeport, Connecticut to improve the water supply, bring gas lighting to streets, and enforce liquor and prostitution laws. He was instrumental in starting Bridgeport Hospital, founded in 1878, and was its first president.

Barnum enjoyed what he publicly dubbed “profitable philanthropy”. “If by improving and beautifying our city Bridgeport, Connecticut, and adding to the pleasure and prosperity of my neighbors, I can do so at a profit, the incentive to ‘good works’ will be twice as strong as if it were otherwise.” He was appointed to the Board of Trustees to Tufts University prior to its founding, and he made several significant contributions to the fledgling institution, including a gift of $50,000 (equivalent to $1,371,964 in 2019) in 1883 to establish a museum (later known as Barnum Museum of Natural History) and hall for the Department of Natural History. Tufts made Jumbo the elephant the school’s mascot, and Tufts students are known as “Jumbos”.

Barnum co-founded the Bridgeport & Port Jefferson Steamboat Company in 1883 with Charles E. Tooker, which continues to operate across the Long Island Sound between Port Jefferson, New York and Bridgeport. The company owns and operates three vessels, one of which is named the M.V. PT Barnum. The Barnum Museum in Bridgeport houses many of Barnum’s oddities and curiosities.

Barnum died from a stroke at home in 1891. He is buried in Mountain Grove Cemetery, Bridgeport, Connecticut, a cemetery that he designed.

At his death, critics praised Barnum for good works and called him an icon of American spirit and ingenuity. He asked the Evening Sun to print his obituary just prior to his death so that he might read it. On April 7, 1891, Barnum asked about the box office receipts for the day; a few hours later, he was dead.

In 1893, a statue in his honor was placed by his former partners James Bailey, James A. Hutchinson, and W. W. Cole, at Seaside Park in Bridgeport. Barnum had donated the land for this park in 1865. His circus was sold to Ringling Brothers on July 8, 1907, for $400,000 (about $10.45 million in 2017 dollars). The Ringling Brothers and Barnum & Bailey circuses ran separately until they merged in 1919, forming the Ringling Bros. and Barnum & Bailey Circus.

The city of Bridgeport issued a commemorative coin in 1936 for their centennial celebration, with his portrait for the obverse. Cartoonist Walt Kelly grew up in Bridgeport and named a character in Barnum’s honor in his Pogo comic strip. An annual six-week Barnum Festival was held for many years in Bridgeport as a tribute to Barnum. The Bethel Historical Society commissioned a life-sized sculpture to honor the 200th anniversary of his birth, created by local resident David Gesualdi and placed outside the public library. The statue was dedicated on September 26, 2010.