Sitting by designation of the Ninth Circuit Court of Appeals, in 1999 Illston wrote the panel decision in DiLoreto v. Downey Unified School District Board of Education, 196 F.3d 958 (9th Cir.

denied, 529 U.S. 1067 (2000), which held that an athletic fence which a public high school made available for commercial advertising is a nonpublic forum from which religious messages could be excluded without violating the First Amendment.

[9] In October 2009, Illston ruled in favor of environmental groups, including the Center for Biological Diversity, that sued the U.S. Bureau of Land Management (BLM) over a 5,000 mile expansion of off-roading trails in California's Mojave Desert.

Illston found that the BLM had violated its own regulations[10] when it designated the routes in 2006[11] without adequately analyzing the impacts on air quality, soils, plant communities and sensitive species such as the endangered Mojave fringe-toed lizard.

Illston called the BLM's plan "flawed because it does not contain a reasonable range of alternatives" to limit damage to sensitive habitat and pointed out that the desert and its resources are "extremely fragile, easily scarred, and slowly healed.

[17] She granted Sony permission to track as much information as possible about those who had seen a private YouTube video about the jailbreak and to read their comments, plus obtain access to IP addresses, accounts, and other details of visitors to sites run by Geohot.

[22][23] in April 2020, Illston issued an order—ultimately overturned by the US Supreme Court—denying Slack Technologies’ motion to dismiss a securities class action complaint against it following a direct listing by the company.

It noted in its unanimous June 2023 decision that lower federal courts had held since the 1960s that liability under Section 11 of the Securities Act of 1933 attaches "only when a buyer can trace the shares he has purchased to a false or misleading registration statement.

"[26] It held that "because we think the better reading of the particular provision before us requires a plaintiff to plead and prove that he purchased shares traceable to the allegedly defective registration statement, we vacate the Ninth Circuit’s judgment holding otherwise.