Writing for the Court, Justice Ruth Bader Ginsburg held that the plaintiffs had standing to sue and that the case hadn’t been rendered moot by the defendant’s voluntary compliance or facility closure. Ginsburg began by addressing the injury-in-fact requirement, distinguishing between injury to the environment and injury to the plaintiff. Ginsburg argued that the relevant inquiry for standing is whether the challenged conduct has caused the plaintiff a concrete injury, rather than whether it has caused a “discernible” injury to the environment. She found that the affidavits from local residents who stopped using the river for recreational and aesthetic purposes due to reasonable concerns about mercury pollution were sufficient to establish a personal injury that’s concrete and particularized.

Regarding the issue of mootness, Ginsburg explained that a defendant’s voluntary cessation of a challenged practice doesn’t automatically deprive a federal court of its power to determine the legality of that practice. Ginsburg established a “stringent” standard for mootness in cases of voluntary cessation, stating that a defendant must show that it’s “absolutely clear that the allegedly wrongful behavior could not reasonably be expected to recur.” Ginsburg noted that because Laidlaw still retained its permit to operate and had only closed the facility during the appeals process, the company hadn’t met this heavy burden.

Ultimately, the Court ruled that the civil penalties authorized by the Clean Water Act served a valid purpose of deterring future violations, which provided a form of redress for the plaintiffs' injuries. Ginsburg concluded that the case remained a live controversy because the threat of future pollution wasn't entirely eliminated. Consequently, the Court reversed the appellate decision and remanded the case for further proceedings.