Rising inequality in the United States has raised concerns about potentially widening gaps in educational achievement by socio-economic status (SES). Using assessments from LTT-NAEP, Main-NAEP, TIMSS, and PISA that are psychometrically linked over time, we trace trends in SES gaps in achievement for U.S. student cohorts born between 1961 and 2001. Gaps in math, reading, and science achievement between the top and bottom quartiles of the SES distribution have fallen by 0.05 standard deviations per decade over this period. The findings are consistent across alternative measures of SES and subsets of available tests and hold in more recent periods. At the current pace of closure, the achievement gap will not be cut in half until the end of the 21st Century.