System and method for automatic versioning and data mutation for user-defined data types. A program development environment may be operable to: create a first version of a data type definition in response to user input; instantiate and store an object of the first version of the data type, optionally including a version identifier indicating the version of the object; modify the first version of the data type definition in response to user input, thereby creating a second version of the data type definition; load the object from storage; and automatically (without user intervention) modify the object to conform to the second version of the data type definition. Successive modifications of the data type definition may generate successive versions of the data type. Objects instantiated and stored according to a previous version of the data type may be loaded from storage and automatically modified to conform to a specified subsequent version.