Prior to the executive order that mandated mask-wearing on federal buildings, mask-wearing policies varied across different agencies as there was no single mandate.
From this article by Bloomberg Law (Aug 2020):
The Trump administration has allowed individual agencies to determine how and when to un-shutter their facilities. The White House Office of Management and Budget encouraged agency leaders to review guidance from the Centers for Disease Control and Prevention, which has recommended face coverings.
Multiple agencies including the Office of Personnel Management, which acts as the government’s central human resources branch, have issued more detailed guidance for bringing staff back to the office, and some have encouraged or even required masks. OPM’s guidance for its own staff doesn’t require mask-wearing.
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Staff have their temperatures checked when entering the building, an HHS spokesperson said. The National Treasury Employees Union, which represents HHS workers, said the agency has required masks to enter or walk through the building, but not in designated workspaces.
As such, it would be difficult to answer precisely which federal buildings or lands had required mask-wearing since it was up to the discretion of each individual agency to implement mask-wearing rules and enforce them.
Furthermore, statewide mask mandates issued by governors / mayors do not cover federal buildings:
The D.C. regulations do not cover federal property, meaning the White House was technically exempt, but the fallout has left city officials scrambling over how to respond.
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The D.C. government is in a difficult position regarding the White House, however, because federal properties do not fall under local regulations.
President Biden's executive order institutes a blanket mandate across all federal agencies to make mask-wearing mandatory.
(a) The heads of executive departments and agencies (agencies) shall immediately take action, as appropriate and consistent with applicable law, to require compliance with CDC guidelines with respect to wearing masks, maintaining physical distance, and other public health measures by: on-duty or on-site Federal employees; on-site Federal contractors; and all persons in Federal buildings or on Federal lands.