Interspecific interactions shape how and when species (and population) ranges change. Natural enemies (like parasites) can slow population spread, or, conversely, a population can ‘outrun’ its enemies and spread uninhibited. Yet, less is know about how mutualistic interactions shape population spread, and what role ‘outrunning’ mutualistic partners plays. Here, I examine host-symbiont interactions specifically (where a symbiont species lives in/on a host species); common across animals and plants, and spanning the spectrum from parasitism to mutualism. I develop a model to determine when a symbiont shapes its host’s population spread versus when the host outruns its symbiont. I find that symbiont transmission mode is key. For density-dependent transmission, symbionts cannot be sustained at the low-density population edge and the host outruns its symbiont, whereas frequency-dependent transmission leads to symbionts affecting host spread. However, this pattern breaks down in the presence of a host Allee effect; spread dynamics switch from ‘pulled’ to ‘pushed’, enabling a symbiont to influence population spread from behind the range edge. Overall, mutualistic symbionts speed up (and parasitic symbionts slow down) host population spread. These findings indicate that contact structures within a population (which shape symbiont transmission) are critical for determining whether host-symbiont interactions influence population spread.