The Late Glacial and early Holocene (ca. 15,000–6,000 cal BP) witnessed major changes in the environmental conditions which led to the establishment of temperate vegetation and animal species, thereby offering new subsistence opportunities to the population of hunter-gatherers. Measurements of the relative abundances in 13C and 15N were applied to large herbivores from northern France to document the change in their habitat. During the early Holocene, red deer show a decrease in δ13C values most likely reflecting the effect of a dense canopy and an increase in δ15N values probably linked to the increased soil activity of soils in foraged territories. Aurochs and roe deer δ13C values also revealed a more densely forested habitat at the end of the Preboreal, while the δ13C values of the wild boar indicate dependence on fruits and underground tubers that were not affected by the canopy effect. Three human individuals from Val-de-Reuil and La Chaussée-Tirancourt dated to the Preboreal period provided relatively high δ15N values when compared with the local fauna and other early Mesolithic humans, which might have resulted from the consumption of freshwater resources especially at Val-de-Reuil. The δ34S values appear to depend more on the geographical location of the individual, as demonstrated by the difference among wild boar δ34S values between sites, rather than related to the protein source of the diet, namely, terrestrial versus aquatic. Our results confirm the influence of the forest ecosystem on the environment and diet of the considered early Mesolithic human of northern France, while the possible contribution of the aquatic ecosystem still needs to be documented.