Place potatoes in a saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 20 minutes or until tender. Drain; return potatoes to pan.
Heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add leek and garlic; cook 2 minutes or until tender, stirring occasionally.
Add milk and butter to potatoes; beat with a mixer at medium speed until smooth. Stir in leek mixture, salt, and pepper. Serve warm.