Which U.S. counties had the largest increases in unauthorized immigrants between 2021 and 2023?

1. Why county-level answers are not in the supplied reporting

The Migration Policy Institute has produced detailed county profiles and an interactive county data hub for the 135 counties with the largest unauthorized populations, and those profiles are the source to consult for precise county counts or changes, but the excerpts provided do not include a ranked list or table of the counties that experienced the largest numeric increases between 2021 and 2023, so the present reporting cannot deliver the exact county rankings the question asks for ^{[1] [2] [6]}.

2. What the national and state-level sources do show about 2021–23 growth

Pew Research Center and related analysts document a sharp nationwide rise in the unauthorized immigrant population—from roughly 10.5 million in 2021 to roughly 14 million in 2023—driven largely by arrivals with some temporary protections (parolees and asylum seekers) and by methodological revisions to Census estimates; this surge occurred across many states, with California, Texas, Florida, New York, New Jersey and Illinois accounting for a large share of the total in 2023 ^{[5] [3] [4]}.

3. Where the largest county increases are most likely to be found, given the evidence

While the sources here do not list top county increases, they make clear that the growth was geographically broad and that states with the largest unauthorized populations in 2021 and 2023—especially California and Texas—are the most plausible locations for the biggest county-level gains; MPI’s top-county profiles (the 135 counties) are explicitly designed to reveal which counties host the largest unauthorized populations and would be the place to extract change scores for 2021–23 ^{[1] [2] [7]}.

4. Methodological caveats that affect any county ranking

Estimates for 2021–23 have been materially affected by revised Census Bureau population accounting and by policy-driven inflows that altered the composition of the unauthorized population (for example, parole programs and asylum flows), meaning that increases reflect both real migration and definitional/measurement shifts; analysts warn that these factors complicate direct comparisons and that MPI and Pew use different methods and weighting strategies that can produce differing county or state results ^{[8] [5] [3]}.

5. How to get the definitive county list and what to watch for

To obtain the exact counties with the largest numeric increases, consult MPI’s county data hub and the downloadable spreadsheets for the 135 counties MPI profiles—those tools were created to present county-level totals and demographic detail and are the authoritative source cited repeatedly in the reporting here ^{[1] [6]}. When extracting county rankings, users should note which dataset and imputation method are used (MPI’s imputation of legal status versus Pew’s estimates built from Census adjustments) because choice of source will change the ranking and magnitude of increases ^{[2] [5]}.

6. Competing narratives and hidden agendas to be mindful of

Different research centers and political actors selectively emphasize either the raw numeric rise or the role of temporary parole/asylum protections and Census adjustments to support policy narratives—advocates may highlight humanitarian and labor-market features of new arrivals while critics may stress scale and enforcement implications—so any county list taken out of context can be weaponized unless accompanied by methodological notes explaining measurement drivers ^{[3] [5]}.