Molybdenum isotopes are fractionated by Earth‐surface processes and may provide a tracer for the recycling of crustal material into the mantle. Here, we examined the Mo isotope composition of arc lavas from Martinique in the Lesser Antilles arc, along with Cretaceous and Cenozoic Deep Sea Drilling Project sediments representing potential sedimentary inputs into the subduction zone. Mo stable isotope composition (defined as δ98Mo in ‰ deviation from the NIST 3134 standard) in lavas older than ∼7 million years (Ma) exhibits a narrow range similar to and slightly higher than MORB, whereas those younger than ∼7 Ma show a much greater range and extend to unusually low δ98Mo values. Sediments from DSDP Leg 78A, Site 543 have uniformly low δ98Mo values whereas Leg 14, Site 144 contains both sediments with isotopically light Mo and Mo‐enriched black shales with isotopically heavy Mo. When coupled with published radiogenic isotope data, Mo isotope systematics of the lavas can be explained through binary mixing between a MORB‐like end‐member and different sedimentary compositions identified in the DSDP cores. The lavas older than ∼7 Ma were influenced by incorporation of isotopically heavy black shales into the mantle wedge. The younger lavas are the product of mixing isotopically light sedimentary material into the mantle wedge. The change in Mo isotope composition of the lavas at ∼7 Ma is interpreted to reflect the removal of the Cretaceous black shale component due to the arrival of younger ocean crust where the age‐equivalent Cretaceous sediments were deposited in shallower oxic waters. Isotopic fractionation of Mo during its removal from the slab is not required to explain the observed systematics in this system. Key Points Mo isotopes are useful tracers for subducted sediment contributions to arc magmas Secular changes in the Mo isotope composition of arc lavas from Martinique reflect changes in the isotope composition of incoming sediment Mo isotopic fractionation during removal of Mo from the subducting slab is not required to explain isotopic systematics of the arc lavas