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By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.