The son of a hardware merchant, his ancestors included a number of sea captains and Revolutionary War patriots. Through his paternal grandfather, held ties with the Hunt Family, including the Boston painter William Morris Hunt.
After demonstrating an aptitude for drawing during his youth, Hassam became apprenticed to a Boston wood engraver in 1876. He was soon employed locally as a freelance illustrator, becoming a well-known figure in the profession. His first noncommercial work was in watercolor, a medium that he would continue to favor for the rest of his career.
Hassam made his first trip to Europe in 1883. Returning to Boston, he exhibited sixty-seven watercolors at the galleries of Williams & Everett to much critical acclaim. He then began to paint the streets and parks of Boston, often depicting them under rainy or overcast skies. At that time, he employed an essentially tonalist palette of quiet browns and greys.
Hassam made a second trip abroad in 1886, spending most of the next three years in Paris, where he showed successfully at a number of exhibitions, including the Paris Salon, the Exposition Universelle (where he received a bronze medal) and at the noted Galerie Georges Petit.
Returning to America in the fall of 1889, Hassam settled in New York City. He continued to pursue his interest in Impressionism, quickly developing a style characterized by brilliant light, vivid color and shimmering brushwork yet still reflecting his concern for descriptive realism.
Throughout the course of his career, Hassam explored such subjects as New York street scenes, New England landscapes, interior genre subjects and during the first world war, his famed series of flag paintings. In 1904, 1908 and 1914, he made trips to the American West, painting in California and Oregon, where he was deeply inspired by the clear blue skies and the rolling landscape. In some of his later work, he began to experiment with pure color, simplified compositions and elongated brushwork, reflecting his awareness of Post-Impressionist design principles. After 1915, he developed an interest in printmaking, etching in particular. He quickly acquired a firm command of graphic techniques, which he pursued with skill and acumen, for the rest of his life.
In 1898, he helped found the Ten American Painters, a group of artists who seceded from the Society of American Artists in order to show their work in small, non-juried exhibitions held annually form 1898 until 1918. The membership consisted of such eminent painters as John Henry Twachtman, William Merritt Chase, J. Alden Weir and Thomas Dewing. Hassam was also affiliated with the New York Water Color Club, which he helped establish, the American Water Color Society and the Pastel Society of New York. He was a regular contributor to the exhibitions of the National Academy of Design, the Pennsylvania Academy of Fine Arts and the Carnegie International.
Childe Hassam made East Hampton, Long Island, his permanent summer residence in 1919. He died there in 1935.