The aim of this research was to explore how preschool-aged children with phonological impairment (PI) realise grammatical morphemes across different phonological contexts (i.e. singleton consonant, consonant cluster, syllable), conditions of finiteness and individual morpheme types. Factors accounting for children’s realisation of grammatical morphemes were also examined. Eighty-seven Australian English-speaking preschoolers (aged 4–5 years) with PI completed the Children’s Assessment of Morphophonology (CHAMP)—an elicited response task—in addition to standardised tests of speech and receptive language. The most challenging grammatical morphemes were finite morphemes (particularly past tense) and grammatical morphemes realised in consonant clusters. The ability to produce consonant clusters in single words significantly accounted for children’s ability to realise grammatical morphemes, regardless of whether grammatical morphemes were realised in singleton, consonant cluster or syllable contexts. Realisation of grammatical morphemes by preschoolers with PI is influenced by phonological and morphological factors. The findings have implications regarding the assessment and differential diagnosis of preschoolers with concomitant phonological and language difficulties.