The time course of Mg²⁺ block and unblock of NMDA receptors (NMDARs) determines the extent they are activated by depolarisation. Here, we directly measure the rate of NMDAR channel opening in response to depolarisations at different times after brief (1 ms) and sustained (4.6 s) applications of glutamate to nucleated patches from neocortical pyramidal neurons. The kinetics of Mg²⁺ unblock were found to be non-instantaneous and complex, consisting of a prominent fast component (time constant ~ 100 µs) and slower components (time constants 4 and ~ 300 ms), the relative amplitudes of which depended on the timing of the depolarising pulse. Fitting a kinetic model to these data indicated that Mg²⁺ not only blocks the NMDAR channel, but reduces both the open probability and affinity for glutamate, while enhancing desensitisation. These effects slow the rate of NMDAR channel opening in response to depolarisation in a time-dependent manner such that the slower components of Mg²⁺ unblock are enhanced during depolarisations at later times after glutamate application. One physiological consequence of this is that brief depolarisations occurring earlier in time after glutamate application are better able to open NMDAR channels. This finding has important implications for spike-timing dependent synaptic plasticity (STDP), where the precise (millisecond) timing of action potentials relative to synaptic inputs determines the magnitude and sign of changes in synaptic strength. Indeed, we find that STDP timing curves of NMDAR channel activation elicited by realistic voltage waveforms are narrower than expected assuming instantaneous Mg²⁺ unblock, indicating that slow Mg²⁺ unblock of NMDAR channels makes the STDP timing window more precise.