The United States experienced a broad reallocation of economic activity across sectors in recent decades. The manufacturing sector accounted for about a quarter of total value added in 1990, but now comprises less than 15%. At the same time, the firm entry rate fell dramatically from about 15% in 1990 to under 9% in 2019. This paper investigates whether and how these two important macroeconomic trends are related. Using data from the Business Employment Dynamics (BED) program of the Bureau of Labor Statistics (BLS), I first document several facts about establishment dynamics over the life-cycle. Notably, average establishment size at birth declined in recent cohorts, especially in the manufacturing sector, while cohort-specific growth and exit rates remained largely unchanged. Then, I build a multi-sector firm dynamics model in which within-sector shocks to firm fundamentals spill over across sectors, shifting the equilibrium composition of demand for sectoral goods. The model features a realistic firm life cycle that can capture the empirical patterns of firm birth, death, and growth within sectors. I use the model to quantify and decompose the contributions of within-sector adjustments in firm dynamics - driven by changes in fixed costs of production, fixed costs of entry, and the persistence and dispersion of idiosyncratic shocks to productivity/demand - versus across-sector shifts in demand to aggregate sectoral reallocation. I find that absent changes in sector-level firm fundamentals, the decline in manufacturing value added would have been more than twice as large, indicating that within-sector firm dynamics attenuated structural change.