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The contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P‐to‐s receiver functions (RFs; using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990–2019. We exploit a recent high‐resolution African continental P‐wavespeed model to migrate our RFs to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (∼100–150 K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ∼1,025 km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar. Plain Language Summary: The African plate has experienced widespread Cenozoic hotspot and rift‐related activity during the development of the East African Rift, the Cameroon Volcanic Line, and other magmatic provinces, for example, central Madagascar. Warmed by the Earth's core, hot mantle rock rises slowly over millions of years within mantle plumes, and melts upon reaching the near‐surface. Mantle plumes may therefore be partly responsible for the formation of volcanoes and crustal magmatism across Africa. However, the number, size, and physical properties of African mantle plumes are debated because geochemical and seismic imaging techniques often used to investigate them are typically sensitive to temperature or mineral chemistry. Instead, we measure the depth at which seismic waves undergo conversions from compressional‐to‐transverse that reflect where mantle minerals undergo changes in their crystal structure. This informs us of variations in both temperature and mineral chemistry in the mantle because both factors affect the precise crystal rearrangement depths. Mantle plumes rooted below 1,000 km depth may exist below East Africa and perhaps also, Cameroon, and Madagascar. Specifically, warm mantle rising below Kenya and Madagascar may transport material from chemically distinct regions on the core‐mantle boundary. Plumes elsewhere are likely purely thermal in nature, not sampling chemically distinct deep mantle regions. Key Points: We investigate mantle transition zone and mid mantle discontinuity structure below Africa using P to s receiver functionsA thin transition zone underlies East African Cenozoic magmatism implying influence of processes at mantle transition zone depths or belowA thermochemically distinct upwelling underlies Kenya while a purely thermal upwelling underlies Ethiopia [ABSTRACT FROM AUTHOR]