Additive manufacturing (AM) provides a higher degree-of-freedom manufacturing process for high-mix low-volume manufacturing. However, the as-built surface quality by AM processes is generally inferior to that fabricated by conventional subtractive manufacturing and the machinability investigation for post-processing of additively manufactured (AMed) parts is insufficient. In this paper, milling experiments are undertaken to improve the surface finish of the AMed high-strength maraging steel (18Ni300) manufactured by additive manufacturing with and without heat treatment. The influence of microstructure on the machinability is explored, including microhardness, cutting force, surface roughness, tool wear, and chip formation. Significant variation in machinability is identified between the AMed samples that feature distinct microstructures. Surface microhardness of the as-built and heat-treated samples both increased after milling. Cutting forces and tool wear increased sharply after ageing treatment but only a minor change was observed in the as-built samples and those subjected to solution treatment. Roughness value of the as-built samples was reduced from ∼10 to <0.4 μm after milling. Ageing treatment induced chip adhesion on the tool surface and high degree of chip curling. According to the chip morphology analysis, machining of both as-built and solution-treated samples will produce smaller chip serrations and continuous chip formation comparing to the large serrations and fracture morphology in ageing-treated chips. This paper elucidates the relationship between material microstructure and machinability of AMed maraging steel.