Background: Late language emergence is a risk indicator for developmental language disorder. Parent-implemented early language intervention programmes (parent programmes) have been shown to have positive effects on children's receptive and expressive language skills. However, long-term effectiveness has rarely been studied. Additionally, little is known about which strategies parents learn to use after participating in parent programmes and whether this affects their child's language development. Aims: To evaluate medium- and long-term effectiveness (1 and 2 years after inclusion) of a low-dosage parent programme in a sample of late talkers (LTs) with an expressive language delay. Specifically, we investigated which strategies the parents learned to use in interaction with their child and which strategies were associated with child language growth over time. Methods & Procedures: This quasi-experimental study with a longitudinal design included 24-month-old LTs. After the pre-test, parents of children in the intervention group received a parent programme (n = 30), while parents of children in the comparison group received care as usual (n = 30). Children's language development was assessed using standardized language tests at pre-test and two follow-up tests at 36 and 48 months of age. Change in parental communicative behaviour was measured by a parent–child interaction observation measure, at pre-test and follow-up at 36 months of age. Outcomes & Results: At 36 months old, children in the parent programme group showed a significantly higher growth in expressive vocabulary than children in the comparison group. However, differences between the groups were not seen in the long-term, at the 48-month follow-up. In contrast to parents in the comparison group, parents in the parent programme group changed their communicative behaviour positively in two domains: Interaction and Pressure on the child. However, no changes in the domains of Language Stimulation and Responsiveness were observed. Children's growth in expressive vocabulary and expressive syntax was associated with a decrease in the domain of Pressure. Conclusions & Implications: At 4 years of age, the majority of children in both groups achieved expressive vocabulary scores within the normal range. However, the mean score for expressive syntax in both groups remained below that of their peers, and 29% of the children still had expressive language scores below the mean range. Ongoing monitoring of LTs’ language development is necessary in order to make decisions regarding the timing and nature of intervention.