One of the greatest threats to the long-term viability of migrating plant species is the loss of genetic diversity due to founder effects. Populations can expand as a response to climate change, but it is uncertain if long-lived plant species can maintain sufficient genetic diversity at the leading edge of migrating populations. This study uses an expanding Larix decidua population investigated along a chronosequence at landscape (350 ha) and local (0.8 ha) scales to test whether accelerated migration as a result of climate warming has the potential to intensify genetic erosion. Nine SSR markers revealed similar genetic diversity among eight sub-populations along the chronosequence (overall He = 0.73; SE = 0.04). Sub-populations were not genetically differentiated and all sampled individuals (N = 730) formed one major genetic cluster indicating homogenizing gene flow despite spatial genetic structure (SGS) up to 80 m. At the local scale, individuals at the leading edge [early successional sub-population (ESSP), N = 140] and a sub-population at equilibrium [late successional sub-population (LSSP), N = 290] revealed high genetic diversity in largest-sized cohorts. SGS among juveniles occurred up to 30 m in LSSP but there was no structure in ESSP. Accordingly, a maximum likelihood paternity assignment revealed local gene dispersal in LSSP (2–48 m) and intermediate-to-long distance dispersal into ESSP (115–3132 m). The findings indicate intensive mixing of the genes in this expanding population instead of founder effects and support the view that genetic diversity can be maintained in a long-lived species during rapid population expansion driven by climate warming.