A mixed-orientation multi-die (“MOMD”) integrated circuit package includes dies mounted in different physical orientations. An MOMD package includes both (a) one or more dies horizontally-mounted dies (HMDs) mounted horizontally to a horizontally-extending die mount base and (b) one or more vertically-mounted dies (VMDs) mounted vertically to the horizontally-extending die mount base. HMDs may include FPGAs or other high performance chips, while VMDs may include low performance chips and other physical structures such as heat dissipators, memory, high voltage/analog devices, sensors, or MEMS, for example. The die mount base of an MOMD package may include structures for aligning and mounting VMD(s), for example, VMD slots for receiving each mounted VMD, and VMD alignment structures that facilitate aligning and/or guiding a vertical mounting of each VMD to the die mount base. MOMD packages may provide a reduced lateral footprint and increased die integration per unit area, as compared with conventional multi-die packages.