Mattituck is located at 40°59'27" North, 72°32'3" West (40.991111, -72.534444).
According to the United States Census Bureau, the city has a total area of 10.24 mi2 (26.51 km2). 8.64 mi2 (22.38 km2) of it is land and 1.59 mi2 (4.13 km2) of it is covered by water. The total area is 15.56% water.
Mattituck is located in the Eastern Standard Time (EST) timezone, GMT-05:00. It observes Daylight Saving Time (DST).