After graduating, Sachs went to work in the family business, becoming a partner in 1904. He married Meta Pollak. When Forbes succeeded Charles H. Moore (q.v.) as the director of the William Hayes Fogg Art Museum in 1909, Forbes looked around for a competent person to be his assistant director.
Sachs had been making donations to the Fogg since 1911, then only a small art collection consisting mostly of Italian primitives. In 1912 Sachs was appointed to the museum's Visiting Committee. In 1914 he persuaded Sachs to leave his investment business to become assistant curator, despite Sachs having no curatorial background. Sachs spent that summer in Italy, seeing as much as as he could before his arrival at Harvard in the autumn of 1915.
Sachs' first lectures in art history occurred in 1916-17 at Wellesley College where he had been appointed "Lecturer in Art." He was made an assistant professor in the department of fines arts at Harvard in 1917. Together, Forbes and Sachs formed a team of fundraising, teaching and museum development which set a standard for academic museum direction. The two were so closely associated that Harvard President A. Lawrence Lowell described them as "mendicant Siamese twins."
Forbes took a leave of absence from the museum briefly for war service in World War I. Because of his height (5 feet, 2 inches), Sachs was ineligible for the army, but served as a major with the American Red Cross.
After returning, Sachs was made an associate director of the Fogg in 1923. The year before, he had begun his celebrated course in museum curatorship, Fine Arts 15a, "Museum Work and Museum Problems," known to students as "the museum course." Sachs' business experience helped in teaching administrative skills to his students.
His position as a collector and person of wealth opened doors to private collections for many of his students. He was appointed full professor in 1927. That same year the Fogg moved to new quarters. Sachs hired Smith graduate Agnes Mongan (q.v.) to assist in cataloging the burgeoning Fogg collection.
In 1929 he advised Abby Aldrich Rockefeller to hire one of his students, Alfred H. Barr., jr., (q.v.) to be the first director of the Museum of Modern Art in New York. Sachs also became one of seven founding members of the Museum and gave it its first drawing, a George Grosz portrait of the artist's mother, 1926-27. Sachs set about developing a program of museum education, developing what he termed the "connoisseur-scholar."
One aspect included what was commonly called "the Print Course," a seminar-style analysis of prints and drawings drawn largely from Sach's personal collection. From 1935 onward, he served regularly as chair of the Division of Fine Arts as the department was then known. In 1936, Sachs participated in the celebrated "Albertina Affair."
Archduke Albrecht, in a bid to gain the title of Emperor of Hungary, attempted a secret negotiation with the Boston Museum of Fine Arts to sell his print and drawing collection--the greatest one in the world and part of the cultural legacy of Austria. Sachs and Mongan traveled secretly to Vienna with Boston curator of Prints and Drawing's Henry Rossiter (q.v.), authenticating hundreds of drawings until the Austrian government learned of the plan.
The collection was seized and nationalized, however, Sachs had nearly acquired the greatest single collection of drawings in the world. Harvard awarded him an honorary doctorate in 1942. In 1945 Sachs and Forbes retired together from the museum, Sachs remaining in the department until 1948 when he was named a professor emeritus. In 1961 his wife preceded him in death.
He died at his desk at Shady Hill while working on his memoirs.
Sachs' contribution to art museology was his famous "Museum Course" a seminar conducted Mondays out of his home, Shady Hill (the former residence of Charles Eliot Norton, q.v.) in Cambridge and Fridays at the Fogg. The class amounted to a detailed connoisseur-style discussion of art. The course was one of the earliest ones in museum studies and through it Sachs trained a great many of the next generation of museum directors.
During the era when art museums were being founded or reestablished as serious institutions, a great many board trustees contacted Sachs for a recently graduated student to head their institution. Universities creating departments of art history also found Sachs willing to outline programs to fit academic requirements. He was an editor of the Art Bulletin from 1919-1940.
Sachs was the kind of person about whom legends quickly arose. Due to his short stature, he hung the paintings at the Fogg to fit his own view. As many of Sach's students eventually were responsible for installations in major museums, they all tended to hang pictures low, assuming Sach's habit to be the museum norm. Students called him "Uncle Paul" for his avuncular countenance--but never to his face. Others, notably Julian Levy, characterized him as "pompous and willful." (Marquis)