Background: Abnormal serum levels of magnesium, phosphate, and zinc appear common in intensive care unit (ICU) patients, but the epidemiology, management, and associations with outcomes are less well described. We described these factors and estimated associations with outcomes in a large dataset of Danish ICU patients. Methods: We included adults who were acutely admitted to 10 general ICUs in Denmark between October 2011 and January 2018. From the dataset, we obtained characteristics of patients who had serum levels measured of magnesium, phosphate, or zinc, including data on supplementation. We used joint models with death as a competing outcome to estimate the associations between abnormal serum levels and time to successful extubation and, for magnesium, also incident tachyarrhythmia. Results: We included 16,517 of 36,514 patients in the dataset. The cumulative probability of hypomagnesemia within 28 days was 64% (95% confidence interval [CI] 62–66); of hypophosphatemia 74% (95% CI 72–75); and of hypozincemia 98% (95% CI 98–98). Supplementation of magnesium was used in 3554 out of 13,506 (26%) patients, phosphate in 2115 out of 14,148 (15%) patients, and zinc in 4465 out of 9869 (45%) patients. During ICU stay, 38% experienced hypermagnesemia, 58% hyperphosphatemia, and 1% hyperzincemia. Low serum levels of magnesium, phosphate, and zinc were associated with shorter time to successful extubation, and high serum magnesium and phosphate and low serum zinc with the competing risk of increased mortality, but too few serum measurements made the results inconclusive. Conclusion: In this multicenter cohort study of acutely admitted ICU patients, most experienced low serum levels of magnesium, phosphate, or zinc during ICU stay, many received supplementation, and experiencing both low and high serum levels during ICU stay was not uncommon. Associations between serum levels and clinical outcomes appeared inconclusive because the data proved unfit for these analyses.