This is an excerpt from an Article regarding Freezing Mint Leaves.
Rinse the leaves in a sink and pat them dry. All herbs, plants and fruit should be washed under running water before cooking with them.
Step 2
Place whole or chopped up leaves in an ice cube tray, and add water. Ideally you should fill each slot in the tray halfway with mint leaves to allow an ice cube to properly form around the mint. Put this tray in the freezer. Freezing the leaves in ice will retain their natural green color.
Step 3
Remove the mint ice cubes from the tray after they have completely frozen. Put the cubes in the zip-top bag, and put them back in the freezer.
Step 4
Thaw the mint cubes before using them in recipes or foods. The mint ice cubes themselves can also be dropped into cold beverages.