The I = 2 π π scattering length is calculated in fully dynamical lattice QCD with domain-wall valence quarks on the asqtad-improved coarse MILC configurations (with fourth-rooted staggered sea quarks) at four light-quark masses. Two- and three-flavor mixed-action chiral perturbation theory at next-to-leading order is used to perform the chiral and continuum extrapolations. At the physical charged pion mass, we find m π a I = 2 π π = − 0.043   30 ± 0.000   42 , where the error bar combines the statistical and systematic uncertainties in quadrature.