This historical and contemporary assessment of 14-19 attainment and post-16 participation suggests that for the first time in a generation, overall levels of attainment in the English upper secondary (14-19) phase are plateauing. Time-series data has been compiled to present four phases of attainment and participation development since the late 1980s, when the education and training system in England transitioned into a full-time post-16 participation model. Closer analysis of the fourth phase (2012/13-present) shows several plateauing trends of learner attainment and participation that originate in Key Stage 4, but which are now spreading throughout the upper secondary phase. The analysis proceeds to explore the main dynamics of the plateauing phase--qualifications reform; accountability measures; and institutional behaviours--that form a new 'attainment and participation equilibrium'. The article concludes by suggesting that the new trend poses particular threats to middle and lower attainers, who could be disproportionately affected by Conservative-led reform, and to the performance of the English upper secondary system when compared internationally.