The cross‐national intragenerational literature has often analyzed income mobility within short time periods over which mobility might reasonably be thought of as invariant. Here, we argue that a great social transformation—German reunification—abruptly and permanently altered mobility. Using standard measures (with panel data for the western states of Germany and the United States) over the period 1984–2006, we find the conventional result that income mobility is greater in Germany. But when we cut the data into 5‐year windows, we find that income mobility declines significantly over the years immediately following reunification in Germany but not in the United States, using both measures. (JEL J1, J6)