In holding that the Section 3 is preempted by federal law, the court stated that the penalties established by S.B. 1070 for failing to comply with federal alien registration requirements:

The fact that the provision had the same aim as federal law and adopted its substantive standards was not sufficient to save it, the court noted, since a state is completely barred from entering a field that the federal government has reserved for itself.

Regarding Section 5(C)'s misdemeanor penalties for illegal immigrants who work or seek work in Arizona, the court held that the comprehensive regulatory framework established by the Immigration Reform and Control Act of 1986 (IRCA) foreclosed state laws penalizing aliens who seek or engage in unauthorized work.

Most persuasive to the court was the fact that the drafters of the IRCA considered and rejected the imposition of criminal penalties on aliens who seek or engage in unauthorized employment, indicating that Congress considered these penalties to be inconsistent with federal policy. The court noted that the IRCA only expressly preempted state penalties against employers who employ illegal aliens, rather than against the aliens themselves, but emphasized that the presence of one express preemption provision does not bar the ordinary working of conflict preemption principles in other areas.

Although the court acknowledged that Section 5(C) attempted to advance the same overall policy objectives as the IRCA, it stated that a conflict in the means by which an objective is achieved can be as disruptive to Congressional intent as a conflict in overt policy.

The court noted its May 2011 decision in Chamber of Commerce v. Whiting, which recognized that IRCA's express preemption provision permitted states to penalize employers for the employment of unauthorized workers in certain circumstances. The court also distinguished its earlier holding in De Canas v. Bica by noting that the state penalties on the employment of illegal aliens upheld in that case preceded the passage of the IRCA.

The court also held that federal law preempts Section 6, which provides an authorization for state and local officers to make warrantless arrests of persons an officer has probable cause to believe have committed public offenses that would make them removable from the US.

Federal law specifies limited circumstances in which state officers may perform the functions of an immigration officer. The court noted that Section 6, by contrast, attempts to provide state officers with even greater authority to arrest aliens on the basis of removability than is given to federal immigration officers. By authorizing state officers to make these arrests, the provision violates the principle that the removal process is entrusted to federal discretion.

The court rejected Arizona's argument that Section 6 complied with a federal statute allowing state officers to cooperate with the US Attorney General in the removal process, stating that the unilateral decision of state officers to arrest aliens, in the absence of any request from the federal government, does not constitute cooperation.

The court reversed the injunction against Section 2(B), which requires state officers to make a reasonable attempt to determine the immigration status of any person they stop, detain or arrest if reasonable suspicion exists that the person is an illegal alien. The court noted that Congress has encouraged the sharing of information about immigration violations and has enacted a statute barring restrictions on the exchange of information between states and the Immigration and Customs Enforcement on immigration status.

Although opponents of the provision argued that the verification requirements would result in the delay of the release of detainees, the court found that the Section 2(B) could be read to avoid this constitutionally problematic outcome. For instance, state courts could take the requirement that the officer make a "reasonable" attempt to verify status to mean that it would be unreasonable to prolong a stop solely to conduct the immigration inquiry.

In the absence of a definitive interpretation from state courts, the court found, it would be inappropriate to assume that Section 2(B) would conflict with federal law. However, the court left open the possibility of future preemption and constitutional challenges once the law is interpreted and has gone into effect.

Justices Scalia, Thomas and Alito each submitted opinions concurring in part and dissenting in part. Scalia and Thomas would have upheld all four provisions, with Scalia emphasizing states' sovereign rights to exclude persons from its territory, and Thomas finding that, under a preemption test that looks to whether there is conflict in the ordinary meaning of the relevant laws, none of the four provisions clashed with federal law.

Alito agreed with the majority as to Sections 3 and 2(B), but argued that, under De Canas v. Bica, Section 5(C)'s penalties for illegal aliens who work or seek work are a permissible exercise of a state's power to regulate employment. Section 6, Alito noted, is also permissible, as it adds little to the arrest authority that Arizona officers already possess, while any additional authority it does confer is consistent with federal laws requiring that the federal government take into custody criminal aliens.