Hassam was known to all as Childe, a name taken from an uncle. Hassam was born in the family home on Olney Street on Meeting House Hill in the Dorchester neighborhood of Boston, on October 17, 1859. His father, Frederick Fitch Hassam (1825–1880), was a moderately successful cutlery businessman with a large collection of art and antiques. He descended from a long line of New Englanders. His mother, Rosa Delia Hawthorne (1832–1880), a native of Maine, shared an ancestor with American novelist Nathaniel Hawthorne. His father claimed descent from a seventeenth-century English immigrant whose name, Horsham, had been corrupted over time to Hassam. With his dark complexion and heavily lidded eyes, many took Childe Hassam to be of Middle Eastern descent—speculation which he enjoyed stoking. In the mid-1880s, he took to painting an Islamic-appearing crescent moon (which eventually degenerated into only a slash) next to his signature, and he adopted the nickname "Muley" (from the Arabic "Mawla", Lord or Master), invoking Muley Abul Hassan, a fifteenth-century ruler of Granada whose life was fictionalized in Washington Irving's novel Tales of the Alhambra.
Hassam demonstrated an interest in art early. He had his first lessons in drawing and watercolor while attending The Mather School, but his parents took little notice of his nascent talent.
As a child, Hassam excelled at boxing and swimming at Dorchester High School. A disastrous fire in November 1872 wiped out much of Boston's commercial district, including his father's business. Hassam left high school after two years (at age 17), and by 1880 his family had moved to nearby Hyde Park. Despite his uncle's offer to pay for a Harvard education, Hassam preferred to help support his family by working. His father arranged a job in the accounting department of publisher Little, Brown & Company. During that time, Hassam studied the art of wood engraving and found employment with engraver George Johnson. He quickly proved an adept "draughtsman" and produced designs for commercial engravings such as letterheads and newspapers. Beginning to paint artistically, his preferred medium was watercolor, mostly outdoor studies, and around 1879 began creating his earliest oils.
In 1882, Hassam became a free-lance illustrator (known as a "black-and-white man" in the trade), and established his first studio. He specialized in illustrating children's stories for magazines such as Harper's Weekly, Scribner's Monthly, and The Century. He continued to develop his technique while attending drawing classes at the Lowell Institute and at the Boston Art Club, where he took life painting classes.
By 1883, Hassam had exhibited watercolors in his first solo exhibition at the Williams and Everett Gallery in Boston. The following year, his friend Celia Thaxter convinced him to drop his first name and thereafter he was known as "Childe Hassam". He also began to add a crescent symbol in front of his signature, the meaning of which remains speculative, possibly an allusion to his penchant for inferring Middle Eastern or Turkish origins.
Having had relatively little formal art training, Hassam was advised by his friend and fellow Boston Art Club member Edmund H. Garrett to join him on a two-month "study trip" to Europe during the summer of 1883. They traveled throughout the United Kingdom, the Netherlands, France, Italy, Switzerland, and Spain, studying the Old Masters together and creating watercolors of the European countryside. Hassam was particularly impressed with the watercolors of J. M. W. Turner. Sixty-seven of the watercolors that Hassam painted on this trip formed the basis of his second exhibition in 1884. During this period, Hassam taught at the Cowles Art School. He also joined the "Paint and Clay Club", expanding his contacts in the art community, which included prominent critics and "the readiest and smartest of our younger generation of artists, illustrators, sculptors, and decorators—the nearest thing to Bohemia that Boston can boast." Friends found him to be energetic, robust, outgoing, and unassuming, capable of self-mockery and considerate acts, but he could be argumentative and wickedly witty against those in the art community who opposed him. Hassam was particularly influenced by the circle of William Morris Hunt, who like the great French landscape painter Jean-Baptiste-Camille Corot, emphasized the Barbizon tradition of working directly from nature. He absorbed their credo that "atmosphere and light are the great things to work for in landscape painting." In 1885, a noted critic, in part responding to Hassam's early oil painting A Back Road (1884), stated that "the Boston taste for landscape painting, founded on this sound French school, is the one vital, positive, productive, and distinctive tendency among our artists today...the truth is poetry enough for these radicals of the new school. It is a healthy, manly muscular kind of art."
In February 1884, after a courtship of several years, Hassam married Kathleen Maude (or Maud) Doane (born 1861), a family friend. Throughout their life together, she ran the household, arranged travel, and attended to other domestic tasks, but little is known about their private life. By the mid-1880s, Hassam began painting cityscapes; Boston Common at Twilight (1885) was of his first. He joined a few other progressive American artists who were taking to heart the advice of French academic master Jean-Léon Gérôme, who abandoned his traditional subject matter and told his American peers, "Look around you and paint what you see. Forget the Beaux-Arts and the models and render the intense life which surrounds you and be assured that the Brooklyn Bridge is worth the Colosseum of Rome and that modern America is as fine as the bric-a-brac of antiquity." However, one Boston critic firmly rejected Hassam's choice of urban subject matter as "very pleasant, but not art." Although he had shown steady improvement in his oil painting, his watercolors continued provided consistent financial success. He returned with his wife to Paris where in 1886 they were able to engage a well-located apartment/studio with a maid near the Place Pigalle, the center of the Parisian art community. With the exception of fellow American artist Frank Myers Boggs [fr], they lived among the French and socialized little with other American artists studying abroad.
Hassam had moved to France to study figure drawing and painting at the prestigious Académie Julian. He took advantage of the formal drawing classes with Gustave Boulanger and Jules Joseph Lefebvre, but quickly moved on to self-study, finding that "[t]he Julian academy is the personification of routine...[academic training] crushes all originality out of growing men. It tends to put them in a rut and it keeps them in it", preferring instead, "my own method in the same degree". His first Parisian works were street scenes, employing a mostly brown palette. He sent these works back to Boston and their sale, combined with that of older watercolors, provided him with sufficient income to sustain his stay abroad. In the autumn of 1887, Hassam painted two versions of Grand Prix Day, employing a breakthrough change of palette. In this dramatic change of technique, he was laying softer, more diffuse colors to canvas, similar to the French Impressionists, creating scenes full of light, done with freer brush strokes. He was likely inspired by French Impressionist paintings which he viewed in museums and exhibitions, though he did not meet any of the artists. Hassam eventually became one of the group of American Impressionists known as "The Ten".
The completed pictures he sent home also attracted attention. One reviewer commented: "It is refreshing to note that Mr. Hassam, in the midst of so many good, bad, and indifferent art currents, seems to be paddling his own canoe with a good deal of independence and method. When his Boston pictures of three years ago...are compared with the more recent work...it may be seen how he has progressed." Hassam contributed four paintings to the Exposition Universelle of 1889 in Paris, winning a bronze medal. At that time, he remarked on the emergence of progressive American artists who studied abroad but who did not succumb to French traditions:
The American Section...has convinced me for ever of the capability of Americans to claim a school. Inness, Whistler, Sargent and plenty of Americans just as well able to cope in their own chosen line with anything done over here...An artist should paint his own time and treat nature as he feels it, not repeat the same stupidities of his predecessors...The men who have made success today are the men who have got out of the rut.
As for the French Impressionists, he wrote "Even Claude Monet, Sisley, Pissarro and the school of extreme Impressionists do some things that are charming and that will live." Hassam was later called an "extreme Impressionist". His closest contact with a French Impressionist artist occurred when Hassam took over Renoir's former studio and found some of the painter's oil sketches left behind. "I did not know anything about Renoir or care anything about Renoir. I looked at these experiments in pure color and saw it was what I was trying to do myself."