Mirey Brook begins in the town of Warwick, Massachusetts, at the outlet of a wetland at the northern base of Mount Grace. It flows north to the head of Sunny Valley, where it receives Mountain Brook and Kidder Brook from the southeast. Continuing north, the brook enters the town limits of Winchester, New Hampshire, just as the valley changes from steep and narrow to flat and wide. The brook reaches the Ashuelot River at the village of Winchester.
State Route 78 follows Mirey Brook from its confluence with Mountain Brook to its mouth at the Ashuelot.
A major tributary of Mirey Brook is Roaring Brook, which enters from the east near the village of Scotland, New Hampshire.