Degradation processes change the chemical composition of oil and can be affected by the mixing of oil into the environment. Here, a ramped pyrolysis (RP) isotope technique is implemented to investigate thermochemical and isotopic changes in coastal environments impacted by the 2010 BP Deepwater Horizon oil spill (DwH). Marsh sediment determined to contain oil by PAH analysis display relatively low thermochemical stability and depleted stable carbon (13C) and radiocarbon (14C) isotopic signatures. The ability of RP to separate oil from background organic material (OM) is established by high oil composition for pyrolysates evolved at low temperatures, as determined by radiocarbon measurement. Applying the RP isotopic technique to beach sediment, tar, and marsh samples collected over a span of 881 days reveals a predominance of oil in the organic material for up to 881 days and varying rates of degradation. Pyrolysis profiles show that the oil degraded faster where rates of mixing were higher. Observing how oil changes thermochemically over time provides a new perspective on oil degradation and its relationship with mixing.