As part of the evaluation of the French plan for the elimination of measles and rubella, we conducted a seroprevalence survey in 2013 to update seroprevalence data for people aged 18–32. A secondary objective was to estimate measles incidence in this population during the 2009–2011 outbreak and, thereby, assess the completeness of mandatory measles reporting. We used a cross-sectional survey design, targeting blood donors aged 18–32 who had been living in France since 2009 and who came to donate blood at a blood collection site. We included 4,647 people in metropolitan France, 806 people on Réunion Island, and 496 in the French Caribbean. An additional 3,942 individuals were interviewed in the southeastern region of metropolitan France to estimate the completeness of mandatory measles reporting. One of the main findings of this survey is that the proportion of people aged 18–32 susceptible to both measles and rubella infections remained high in France in 2013—9.2% and 5.4%, respectively, in metropolitan France—even after the catch-up vaccination campaigns conducted during and following the major measles epidemic of 2009–2011. Applying our results to French census data suggests that approximately 1 million people aged 18–32 are currently susceptible to measles in France, despite this age group being one of the vaccination targets of the national measles elimination plan. Another important finding is that only an estimated 45% of the actual number of cases in this age group were reported, despite mandatory reporting requirements.