We examined correlates of ever injecting drugs in Mexico among residents of San Diego, California. From 2007 to 2010, injecting drug users (IDUs) in San Diego underwent an interviewer-administered survey. Logistic regression identified correlates of injection drug use in Mexico. Of 302 IDUs, 38% were Hispanic, 72% male and median age was 37; 27% ever injected in Mexico; 43% reported distributive syringe sharing there. Factors independently associated with ever injecting drugs in Mexico included being younger at first injection, injecting heroin, distributive syringe sharing at least half of the time, and transporting drugs over the last 6 months. One-quarter of IDUs reported ever injecting drugs in Mexico, among whom syringe sharing was common, suggesting possible mixing between IDUs in the Mexico-US border region. Prospective studies should monitor trends in cross-border drug use in light of recent Mexican drug policy reforms partially decriminalizing drug possession.