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The early developmental profile of Kleefstra syndrome remains undercharacterized. To address this gap, this study investigated a large clinical cohort of patients with Kleefstra syndrome, characterizing age of achievement of infant/toddler developmental milestones and quantifying language and visual motor developmental quotients (DQs) using the Capute Scales developmental screening tool. We conducted a retrospective chart review on individuals with molecularly confirmed Kleefstra syndrome. We reported age of achievement of motor and language milestones. In a subset of this cohort, we evaluated DQs for language and visual motor skills based on the Capute Scales. Among 100 individuals (43 males, 57 females; median age 9 years), rolling occurred at a median of 6 months, sitting at 10 months, independent walking at 1.96 years, and first words at 24 months. Capute Scales testing (n = 24) showed median DQs as follows: visual motor skills (53, IQR = 42-71), overall language (56, IQR = 42-67), expressive language (52, IQR = 35-60), and receptive language (50, IQR = 42-61). This work quantifies the early developmental profile of Kleefstra syndrome and suggests that developmental delay can be significant from an early age, making early initiation of services such as physical therapy, occupational therapy, and speech therapy crucial to ensuring optimal skills development. This cross-sectional analysis highlights the need for incorporating longitudinal developmental assessments into the clinical care of patients with Kleefstra syndrome-particularly during infancy and early childhood-to ensure that appropriate educational supports are in place.