In 1819, he joined a friend who was sailing to North America. After four weeks they landed in Halifax, Nova Scotia, where Bennett briefly worked as a schoolmaster till he had enough money to sail south to Portland, Maine, where he again taught school in the village of Addison, moving on to Boston, Massachusetts by New Year's Day, 1820. He worked in New England as a proofreader and bookseller before the Charleston Courier in Charleston, South Carolina hired him to translate Spanish language news reports, so he briefly relocated to The South. He moved back north to New York City in 1823, where he worked first as a freelance paper Writer and, then, assistant Editor of the New York Courier and Enquirer, one of the oldest newspapers in the city.

He later endorsed Southerner Democrat and incumbent Vice President John C. Breckinridge (1821-1875, served 1857-1861), of Kentucky under Buchanan for the 1860 presidential campaign, then shifted to John Bell (1796-1869), of Tennessee running as a Constitutional Unionist among the four presidential candidates in the confused but pivotal general election in November 1860. During the midst of the following Civil War (1861-1865), he promoted former Union Army General-in-Chief George B. McClellan (1826-1885), nominated from the Democratic Party in the 1864 Election, campaigning for a negotiated peace with the South against a second term for wartime 16th President Abraham Lincoln (1809-1865, served 1861-1865), but the paper itself endorsed no candidate for the unusual war election of 1864.

In May 1835, Bennett began the New York Herald after years of failing to start a paper. After only a year of publication, in April 1836, it shocked readers with front–page coverage of the grisly murder of prostitute Helen Jewett; Bennett got a scoop and conducted the first-ever newspaper interview for it. In Business and circulation policy, The Herald initiated a cash–in–advance policy for advertisers, which later became the industry standard. Bennett was also at the forefront of using the latest Technology to gather and report the news, and added pictorial illustrations produced from woodcuts. In 1839, Bennett was granted the first ever exclusive interview to a sitting President of the United States, the eighth occupant, Martin Van Buren (1782-1862, served 1837-1841).

On June 6, 1840 he had married Henrietta Agnes Crean in New York. They had three children, including:

By the time Bennett turned control of the New York Herald over to his son James Gordon Bennett Jr. (1841-1912), at age 25 in 1866, it had the highest circulation in America but would soon face increasing competition from Greeley's Tribune and soon in the next decades, from Joseph Pulitzer's New York World, william Randolph Hearst's New York Journal, along with Henry J. Raymond's The New York Times. However, under the younger Bennetts' stewardship, the paper slowly declined under the increasing stiff competition and changing technologies in the late 18th century and, after his 1912 death, it was merged a decade later with its former arch-rival, the New York Tribune in 1924, becoming the New York Herald Tribune for another 42 years meeting with considerable success and reputation in its near last half-century, until finally closing in 1966-1967.

Although he generally opposed Republican Abraham Lincoln, Bennett still backed the Northern cause with the Union, then took the lead to turn the Republican war President into a martyr after his sudden April 14, 1865 assassination at Ford's Theater in Washington. He favored most of successor 17th President Andrew Johnson (1808-1875, served 1865-1869), former Vice President for one month in Lincoln's brief second term, a War Democrat, former unseceded U.S. Senator and loyal wartime Governor of Tennessee, and his following moderate Reconstruction Era policies and proposals towards the defeated South, following what was thought to be would have been President Lincoln's gentle hand had he lived.

He died in Manhattan, New York City, on June 1, 1872. This was five months before his rival / competitor Horace Greeley also succumbed to illness in November 1872, after his disastrous presidential election campaign of 1872. He was interred at Green-Wood Cemetery in Brooklyn, New York City.

Bennett's account of the infamous Helen Jewett murder in the Herald was selected by The Library of America for inclusion in the 2008 anthology True Crime.

He has a street named for him from West 181st Street to Hillside Avenue in the Washington Heights neighborhood of northern Manhattan, and a New York City park, Bennett Park named in his honor along Fort Washington Avenue. The Avenue Gordon Bennett in Paris, France with Stade de Roland Garros, site of the French Open, tennis tournament, is also named after James Gordon Bennett, Sr., possibly thanks to his son.