Abstract The aim of this study was to investigate the relation between a variety of child, parent, family and environmental factors and pre-treatment motivation of parents of children and adolescents newly referred to a mental health care clinic in The Netherlands. Data were collected of 521 parents most involved in the upbringing of the child (443 mothers and 78 fathers; Dutch origin 97.1%) of 207 girls and 314 boys (age M = 10.2, range 1–18 years). Treatment motivation was measured by the Parent Motivation Inventory. Least absolute shrinkage and selection operator (LASSO) regression was used to investigate the prediction strength of 33 factors in 5 domains: (1) source of referral and prior use of healthcare services, (2) child characteristics, (3) characteristics of the primary parent, (4) parenting characteristics of the primary parent, and (5) family characteristics. Twenty-one factors were statistically relevant, explaining 21.3% of the deviance in pre-treatment motivation. Child characteristics, mainly type and severity of problems, contributed most to the model. Notably, internalising problems contributed more than externalising problems. Furthermore, we found relations between parental pre-treatment motivation and parents' perceived self-efficacy, parents' perceived parenting competence, financial problems and source of referral. Our findings provide insight into the multifacetedness of parental motivation prior to starting treatment and inform health professionals of specific contextual factors of interest in parents' readiness to change their behaviour and participate in treatment.