Igneous sheet-complexes transport magma through the crust, but most studies have focused on single segments of the magma transport system or have low resolution. In the Jameson Land Basin in East Greenland, seismic reflection data and extensive outcrops give unparalleled constraints on mafic intrusions down to 15 km. This dataset shows how sill-complexes develop and how magma is transported from the mantle through sedimentary basins. The feeder zone of the sill-complex is a narrow zone below a basin, where a magmatic underplate body impinges on thinned crust. Magma is transported through the crystalline crust through dykes. Seismic data and published geochemistry indicate that magma is supplied from a magmatic underplate without perceptible storage in crustal magma chambers and crustal assimilation. As magma enters the sedimentary basin, it forms distributed, bowl-shaped sill-complexes throughout the basin. Large magma volumes in sills (4–20 times larger than the Skaergaard Intrusion) and the presence of few dykes highlight the importance of sills in crustal magma transport. On scales smaller than 0.2 km, host-rock lithology, and particularly mudstone tensile strength anisotropy, controls sill architecture in the upper 10 km of the basin, whereas sills are bowl-shaped below the brittle–ductile transition zone. On scales of kilometres and towards basin margins, tectonic stresses and lateral lithological changes dominate architecture of sills.