Gait speed modulation, including abruptly decreasing or increasing gait speed, is a challenging task and prerequisite for safe mobility in the community. Older adults with Mild Cognitive Impairment (MCI) exhibit gait deficits under challenging walking conditions which may increase their risk of falls. The purpose of this study was to investigate spatiotemporal variability during slow and fast speed transitions in older adults with and without MCI. Twenty-five older adults with MCI (mean age = 68.56 ± 3.79 years) and 25 cognitively intact controls (mean age = 68.72 ± 4.67 years) participated. Gait performance during gait speed transitions was measured in two walking conditions: 1) a slow to fast speed transition in response to a randomly presented cue, and 2) a fast to slow speed condition in response to a randomly presented cue. Means and variability of spatiotemporal parameters during the transitions were measured and mixed model repeated measures ANOVAs were used to assess interaction and main effects. The older adults with MCI exhibited greater variability of step length (MCI = 13.93 ± 5.38, Control = 11.12 ± 3.15, p = 0.03) and swing time (MCI = 13.35 ± 6.01, Control = 10.43 ± 2.87, p = 0.03) than the controls during the fast to slow speed transitions. No other between-group differences were evident for the gait parameters across the two walking conditions. The findings suggest that older adults with MCI have reduced ability to adapt their gait during transitions from fast to slow walking speeds. This impairment may indicate a decline in automated regular rhythmic gait control and explain in part why this group is at increased risk of falls. Slow speed transition task might be incorporated as a fall risk screening in older adults with MCI.