Garber's family were Mennonite farmers who had migrated to Indiana from Pennsylvania. Despite the asceticism of his way of life and surroundings, his family allowed his inclination to art to develop. He entered the Art Academy of Cincinnati in the summer of 1897, and continued attending day and evening classes in life drawing and illustration for the next two regular seasons, studying with Vincent Nowottny, Otto Walter Beck, and Lewis Henry Meakin. In Cincinnati he was surely exposed to the work of the most prominent artist in the city, Frank Duveneck, and through the Duveneck circle to the Impressionist aesthetic of John Twachtman and Julian Alden Weir. In the summer of 1899 Garber was attending the Darby School of Painting, just established at Fort Washington, Pennsylvania, by Thomas Anshutz and Hugh Breckenridge; Anshutz was especially impressed by his work. From there, he moved on to Philadelphia and night classes in life drawing at the Pennsylvania Academy of the Fine Arts. He continued study at the Pennsylvania Academy into 1905, working primarily under Anshutz.
Around 1901, the year he married fellow student Mary Franklin, Garber established a studio in Philadelphia. While pursuing his studies at night, he was already working professionally. As a commercial artist he designed can labels, decorative headings for newspapers, department store displays, and did illustrations for McClure's, Scribner's, Harper's Bazaar and Century. His paintings were first seen in the exhibitions of the Pennsylvania and the National academies in 1902; at the Society of American Artists in 1903; and in that same year at the Macbeth Galleries in New York, beginning a long association with Macbeth. In 1904 Garber had his first experience as a teacher, giving the life and antique drawing classes at the Philadelphia School of Design for Women.
Garber won the Pennsylvania Academy's Cresson Fellowship in 1905 which provided for two years' study in Europe. By July he was settled in London; November though March 1906 were passed in Italy; from May into the summer of 1907, when he returned to Philadelphia, he was living and working in Paris.
On his return Garber settled on property he called "Cuttalossa" in Lumberville near New Hope in Bucks County, Pennsylvania, which would be his summer home until 1916 when it became his year-round residence. The buildings and gardens of Cuttalossa, and the Lumberville countryside provided Garber with a lifetime of subject matter for his decorative Impressionist landscapes. Unlike most of his colleagues in the New Hope School, however, Garber was also interested in the figure, and frequently posed his wife and daughter in landscapes and interiors.
On his return from Europe, Garber taught for several more years at the Philadelphia School of Design for Women. In 1909 he joined the faculty of the Pennsylvania Academy, first as an assistant to Anshutz; he remained an instructor in the Pennsylvania Academy school through the 1949-50 season. The National Academy awarded Garber the First Hallgarten Prize in the annual exhibition of 1909, the first of the many major prizes given his paintings throughout his lifetime. From the National Academy he also received Altman prizes in the winter exhibitions of 1915 and 1917, and annual exhibitions of 1921, 1922 and 1927; and the Carnegie Prize in the winter exhibition of 1923.
