Transient behaviours during spin-up in rotating Rayleigh–Bénard convection (RBC) with imposed rotation were quantitatively investigated in laboratory experiments. Horizontal and vertical velocity fields were measured by particle image velocimetry with water as the test fluid. Varying the aspect ratio, Rayleigh number and Taylor number, a total of twenty parameters were systematically explored. Toroidal and spiral rolls were formed when the flow reached the rigid-body rotation state, and creation of these structures propagated from the rim towards the internal regions together with the development of the spin-up. Alternate alignments of rolls with opposite meridional circulation transported azimuthal momentum in the rigid-body rotation, and a meandering velocity profile in the radial direction, induced Kelvin–Helmholtz (KH) instability generating azimuthally aligned vortices. The vortices progressively decreased in horizontal dimensions with the wall-to-centre propagation of the vortex formation, but the vortical structures remain larger than the columnar vortices formed in the equilibrium state of a rotating RBC. At the intermediate radius of the fluid layer, the wall-to-centre propagation of the roll formation was overtaken by that of the KH vortex formation. Farther into the interior region, thermal plumes forming columnar vortices were generated as separations of the thermal boundary layers, and the system reached an equilibrium state of rotating RBC dominated by columnar vortices. Use of a fluid vessel with a moderate aspect ratio clarified these transitions to occur in a stepwise fashion, and a spin-up time scale unique in the rotating RBC was found to be from a few to 10 times the Ekman time scale.