Drain pineapple, reserving juice; set the pineapple aside. In a large saucepan, combine pineapple juice and marshmallows; cook and stir over low heat until marshmallows are melted. Remove from the heat.
Dissolve gelatin in boiling water. Add cream cheese; stir until mixture is thoroughly blended. Stir in cold water and marshmallow mixture. Chill until partially set. Fold in carrots and pineapple.
Pour into a 6-cup mold coated with cooking spray. Refrigerate until set. Unmold onto a lettuce-lined serving plate if desired. Garnish with lemon slices if desired.