Objective: As the number of informal caregivers and their caregiving responsibilities increase, this study aims at evaluating caregiver distress, quality of life (QoL) and their predictors in informal caregivers of cancer patients during active treatment and follow‐up. Methods: This cross‐sectional descriptive study targeted primary caregivers of patients with different cancer diagnoses. Caregiver‐reported outcomes were measured by the Caregiver Risk Screen (CRS), Distress Thermometer (DT) and Caregiver Quality of Life Index—Cancer (CQOLC). Results: Caregivers (n = 1580) experienced a low‐to‐moderate risk of caregiver distress and a moderate QoL during both treatment and follow‐up. About 13% reported a high caregiver risk and 20% reported severe distress. There was a strong and significant correlation between caregiver distress and caregivers' QoL (0.793). Predictive factors for higher distress and poorer QoL were: fewer emotional and practical resources, being female, non‐spousal relationship or not living together (p < 0.05). Caregivers of patients with head‐and‐neck, skin, lung and brain cancers reported the highest distress and lowest QoL. Conclusion: Caregiver distress is highly variable, but a minority of caregivers is at high risk for caregiver distress. Professional caregivers play an important role at supporting caregivers and detecting high‐risk caregivers.