We report on results of spectropolarimetry of the afterglow of the long gamma-ray burst GRB 191221B, obtained with SALT/RSS and VLT/FORS2, as well as photometry from two telescopes in the MASTER Global Robotic Network, at the MASTER-SAAO (South Africa) and MASTER-OAFA (Argentina) stations. Prompt optical emission was detected by MASTER-SAAO 38 s after the alert, which dimmed from a magnitude (white-light) of similar to 10-16.2 mag over a period of similar to 10 ks, followed by a plateau phase lasting similar to 10 ks and then a decline to similar to 18 mag after 80 ks. The light curve shows complex structure, with four or five distinct breaks in the power-law decline rate. SALT/RSS linear spectropolarimetry of the afterglow began similar to 2.9 h after the burst, during the early part of the plateau phase of the light curve. Absorption lines seen at similar to 6010 and 5490 angstrom are identified with the Mg II 2799 angstrom line from the host galaxy at z = 1.15 and an intervening system located at z = 0.96. The mean linear polarization measured over 3400-8000 angstrom was similar to 1.5 per cent and the mean equatorial position angle (theta) was similar to 65 degrees. VLT/FORS2 spectropolarimetry was obtained similar to 10 h post-burst, during a period of slow decline (alpha=-0.44), and the polarization was measured to be p = 1.2 per cent and theta = 60 degrees. Two observations with the MeerKAT radio telescope, taken 30 and 444 d after the GRB trigger, detected radio emission from the host galaxy only. We interpret the light curve and polarization of this long GRB in terms of a slow-cooling forward shock.