Major strike-slip faults that develop between strong and weaker regions are thought to focus along narrow shear zones at the rheological boundary. Here we present the first InSAR-derived velocity field spanning almost the entire length of one such fault, the 1600 km-long Altyn Tagh Fault (ATF), and analyse the strain distribution. We find that localisation of strain is actually variable, with strain concentrated at the fault for some sections and distributed over broad (>100 km) shear zones for others. This contrasts to the North Anatolian and central San Andreas faults, where strain rate varies little along the entire length. Slip rate along the ATF is also variable, decreasing along the fault from 11.6 ± 1.0 mm/yr in the west to 7.5 ± 1.2 mm/yr in the central portion, before increasing again to 11.1 ± 1.1 mm/yr over the eastern portion. These results imply that the role of the ATF as a main controlling fault up against a rigid Tarim Basin is smaller than previously thought, with sub-parallel faults playing an important role in the overall deformation. This demonstrates the importance of accurately characterising strain rates over a broad region when assessing seismic hazard.