We analyse social mobility in London and seek to address two paradoxes. Among people living in London, relative mobility, or social fluidity, appears to be remarkably low when compared with other regions of Great Britain. But social fluidity among people who were brought up in London is similar to that of people brought up elsewhere in Britain. This is our first paradox. Furthermore, it is widely held that social fluidity is higher among people with higher levels of education, yet, the proportion of people with a University degree is much higher among people living in London than in any other region: how is this compatible with its relatively low social fluidity? This is our second paradox. We address these puzzles and find that they are largely explained by patterns of migration into and out of London by two groups that have received little attention hitherto in studies of mobility in the capital: International migrants and people who were brought up in London but who no longer live there.