In this study, aberration-corrected scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) was used to investigate the atomistic structure and chemical composition of true twin and order twin boundaries in ferromagnetic τ-MnAl-C. True twins and order twins were distinguished based on the diffraction patterns using TEM. No elemental segregation was observed at the coherent true twin boundary but some Mn enrichment within a region of about 1.5–2 nm was found at the incoherent true twin boundary. A transition region with Mn enrichment about 4–6 nm wide was found at the order twin boundary. A carbon cluster with a size of around 5 nm was also found at the twin boundary. Micromagnetic simulations were conducted to study the effect of this chemical segregation at twin interfaces on the magnetic properties. The results showed that the coercivity tends to increase with increasing structural and chemical disorder at the interface.