Helen Clark Hagar was born on September 8, 1896 in Peabody, Massachusetts, the youngest child of William C. and Emma Florence Hagar. She graduated from Peabody High School in 1915. In the 1920s she attended the George Vesper School of Art, located in Boston. After her graduation, she moved to Salem to live with her mother and aunt and worked in the field restoring antique furniture and other works of art. She lived in Salem until her death in 1984.

Hagar studied, practiced, and taught stenciling and lectured on its history. She was employed first by the Society for the Preservation of New England Antiquities and then by the National Park Service to live in and conduct tours of the Derby House (178 Derby Street, Salem, MA). During this time she sold some of her works in a shop in the Derby House.

Hagar both restored antique household utensils and furnishings and painted new utensils (such as trays) to match the period. Christmas cards, fabrics, object d'art (such as painted ceramics), and the designs of early American stencillers served as sources of inspiration. She primarily used floral and leaf designs, as well as shells and baskets. Helen Hagar also decorated objects for specific people and occasions, such as a Christmas music book. Beverly High School was the site of her painting classes for adults.