The rulings came three days after the Occupational Safety and Health Administration's emergency measure for businesses started to take effect.
The mandate required that workers at businesses with 100 or more employees get vaccinated or submit a negative Covid test weekly to enter the workplace. It also required unvaccinated workers to wear masks indoors at work.
"Although Congress has indisputably given OSHA the power to regulate occupational dangers, it has not given that agency the power to regulate public health more broadly," the court wrote in an unsigned opinion.
"Requiring the vaccination of 84 million Americans, selected simply because they work for employers with more than 100 employees, certainly falls in the latter category," the court wrote.
Liberal Justices Stephen Breyer, Sonia Sotomayor and Elena Kagan dissented, writing that the majority has usurped the power of Congress, the president and OSHA without legal basis.
"In the face of a still-raging pandemic, this Court tells the agency charged with protecting worker safety that it may not do so in all the workplaces needed," they said in their dissent.
"As disease and death continue to mount, this Court tells the agency that it cannot respond in the most effective way possible. Without legal basis, the Court usurps a decision that rightfully belongs to others. It undercuts the capacity of the responsible federal officials, acting well within the scope of their authority, to protect American workers from grave danger," they wrote.