Which U.S. counties gained the most unauthorized residents between 2019 and 2023, and what local factors explain those gains?
Executive summary
Migration Policy Institute analysts estimate the unauthorized immigrant population rose sharply between 2019 and mid‑2023—roughly 3 million additional people nationally—and MPI produced county‑level profiles for the 135 counties with the largest unauthorized populations to document where the growth occurred [1] [2]. Public reporting using MPI’s work highlights Los Angeles, Harris (Houston area) and Cook (Chicago) counties as holding the largest shares of the national unauthorized population, while Florida and Texas registered some of the biggest state‑level increases—facts that point to a mix of long‑standing immigrant hubs plus newer arrival corridors as drivers of county gains [3] [4].
1. Which counties gained the most unauthorized residents: the headline list and the data source
The most authoritative, publicized county‑level picture comes from the Migration Policy Institute’s county profiles and companion fact sheet, which cover the 135 counties with the largest unauthorized populations and underpin media summaries identifying Los Angeles County, Harris County and Cook County among those with the largest shares of the country’s unauthorized residents [2] [3] [1]. MPI’s data tool and profiles are the primary source for county estimates through 2023 and are derived from pooled 2019–23 American Community Survey data combined with SIPP surveys and expert weighting to 2023 totals [2] [5]. The reporting available in the provided sources does not supply a ranked, four‑year list of the single counties that gained the most unauthorized residents by absolute numeric change in 2019–2023 within the snippets; the MPI county data hub is the place to retrieve those precise county‑by‑county change figures [2].
2. Why long‑standing hubs still lead: population base, jobs and networks
Large metropolitan counties such as Los Angeles, Cook and Harris concentrate jobs in construction, hospitality, agriculture and services, contain deep immigrant social networks, and already had large unauthorized populations—conditions that produce both inertia and continued in‑migration because new arrivals follow jobs and kin (MPI’s county profiles document industry and sociodemographic composition that explain settlement patterns) [5] [2]. MPI’s analysis emphasizes that county profiles reveal not only counts but the economic sectors, years‑of‑residence and family structures that sustain those communities, which helps explain why established hubs continued to register large shares of unauthorized residents through 2023 [6] [5].
3. Why Florida and some Sun Belt counties surged: shifting origins, humanitarian parole and secondary migration
Florida showed outsized gains in 2019–2023 in several public summaries and encounter‑based reporting—state totals noted as large in 2021–24 accounting (e.g., Florida +400,000 in a border‑encounter summary)—and the composition of arrivals shifted substantially toward Venezuela, Cuba, Colombia and other Latin American countries, channels that routed many people to Florida’s Miami‑Dade, Broward and other counties [4] [7]. In addition, federal parole programs and spikes in asylum applicants and border encounters in 2022–23 increased arrivals and paroled entries that tend to concentrate in certain metro areas with existing communities and service infrastructure, boosting county totals where those services are present [7] [8].
4. Border dynamics, encounter patterns and timing: why 2019–2023 was exceptional
The national rise between 2019 and mid‑2023 reflects both larger inflows and methodological revisions in population measurement; MPI and Pew note the population grew sharply after a decade of stagnation, and CBP border encounter spikes in 2022–23—compounded by the end of Title 42 in May 2023 and changing enforcement—helped produce the rapid increase in recent years [1] [9] [4]. Because encounters, parole programs and visa overstay dynamics vary by arrival route and time, counties near initial reception points or with strong secondary‑migration pull experienced disproportionate gains [8] [10].
5. Data caveats and what the available sources do and do not show
MPI’s county profiles are the best available public source for county‑level estimates through mid‑2023 and use imputation methods applied to pooled ACS and SIPP data weighted to expert totals, but the snippets provided do not include a simple table of the top counties ranked by absolute gain from 2019 to 2023—those precise per‑county change figures require consulting the MPI county spreadsheet or dataset directly [2] [5]. Independent sources like Pew and university researchers corroborate large national increases and shifting country origins—facts that help explain county patterns—but methodological differences among MPI, Pew and other groups mean exact county rankings can differ depending on estimation choices [7] [9].