Initially, the Main Branch was opened at 1 p.m. on Sundays and 9 a.m. on all other days, and it closed at 10 p.m. each day.
This was to encourage patrons to use the new library.
By 1926, the library was heavily patronized, with up to 1,000 people per hour requesting books.
The library was most used between 10 a.m. to 12 p.m. and 3:30 to 5:50 p.m., and from October through May. The most highly requested books were those for economics and American and English literature, though during World War I geography books were the most demanded because of the ongoing war.
It was estimated that 4 million people per year used the Main Branch in 1928, up from 2 million in 1918 and 3 million in 1926.
There were 1.3 million books requested by nearly 600,000 people through call slips in 1927.
By 1934, though annual patronage held steady at 4 million visitors, the Main Branch had 3.61 million volumes in its collection.

Due to the increased demand for books, new shelves were installed in the stockrooms and the cellars by the 1920s to accommodate the expanded stacks.
However, this still proved to be insufficient.
The New York Public Library announced an expansion of the Main Branch in 1928.
Thomas Hastings prepared plans for new wings near the north and south sides of the structure, which would extend eastward toward Fifth Avenue, as well as a storage annex in Bryant Park to the west.
The expansion was planned to cost $2 million, but was never built.
After Hastings died in 1929, it was revealed that his will contained $100,000 for modifications to the facade, with which he had been dissatisfied.

A theater collection was installed in the Main Reading Room in 1933.
Two years later, the Bryant Park Open-Air Reading Room was established, operating during the summer.
The reading room was meant to improve the morale of readers during the Great Depression, and it operated until 1943, when it closed down due to a shortage of librarians.
In 1936, library trustee George F. Baker gave the Main Branch forty issues of the New-York Gazette from the 18th century, which had not been preserved anywhere else.
In 1937, the doctors Albert and Henry Berg made an offer to the library's trustees to donate their collections of rare English and American literature.
After Henry died, the collection was dedicated in his memory.
The Berg Reading Room was formally dedicated in October 1940.

During the 1930s, Works Progress Administration (WPA) workers helped maintain the Main Branch.
Their tasks included upgrading the heating, ventilation, and lighting systems; refitting the treads on the branch's marble staircases; painting the bookshelves, walls, ceilings, and masonry; and general upkeep.
The WPA allocated $2.5 million for the building's maintenance.
In January 1936, it was announced that the Main Branch's roof would be renovated as part of a seven-month WPA project.
Initially, the Main Branch was opened at 1 p.m. on Sundays and 9 a.m. on all other days, and it closed at 10 p.m. each day. This was to encourage patrons to use the new library. By 1926, the library was heavily patronized, with up to 1,000 people per hour requesting books. The library was most used between 10 a.m. to 12 p.m. and 3:30 to 5:50 p.m., and from October through May. The most highly requested books were those for economics and American and English literature, though during World War I geography books were the most demanded because of the ongoing war. It was estimated that 4 million people per year used the Main Branch in 1928, up from 2 million in 1918 and 3 million in 1926. There were 1.3 million books requested by nearly 600,000 people through call slips in 1927. By 1934, though annual patronage held steady at 4 million visitors, the Main Branch had 3.61 million volumes in its collection.
Due to the increased demand for books, new shelves were installed in the stockrooms and the cellars by the 1920s to accommodate the expanded stacks. However, this still proved to be insufficient. The New York Public Library announced an expansion of the Main Branch in 1928. Thomas Hastings prepared plans for new wings near the north and south sides of the structure, which would extend eastward toward Fifth Avenue, as well as a storage annex in Bryant Park to the west. The expansion was planned to cost $2 million, but was never built. After Hastings died in 1929, it was revealed that his will contained $100,000 for modifications to the facade, with which he had been dissatisfied.
A theater collection was installed in the Main Reading Room in 1933. Two years later, the Bryant Park Open-Air Reading Room was established, operating during the summer. The reading room was meant to improve the morale of readers during the Great Depression, and it operated until 1943, when it closed down due to a shortage of librarians. In 1936, library trustee George F. Baker gave the Main Branch forty issues of the New-York Gazette from the 18th century, which had not been preserved anywhere else. In 1937, the doctors Albert and Henry Berg made an offer to the library's trustees to donate their collections of rare English and American literature. After Henry died, the collection was dedicated in his memory. The Berg Reading Room was formally dedicated in October 1940.
During the 1930s, Works Progress Administration (WPA) workers helped maintain the Main Branch. Their tasks included upgrading the heating, ventilation, and lighting systems; refitting the treads on the branch's marble staircases; painting the bookshelves, walls, ceilings, and masonry; and general upkeep. The WPA allocated $2.5 million for the building's maintenance. In January 1936, it was announced that the Main Branch's roof would be renovated as part of a seven-month WPA project.
New York Public Library Main Branch