  We make the first measurement of the braking index $n$ of two putative
millisecond magnetars born in short gamma-ray bursts. We measure $n=2.9\pm0.1$
and $n=2.6\pm0.1$ for millisecond magnetars born in GRB 130603B and GRB 140903A
respectively. The neutron star born in GRB 130603B has the only known braking
index consistent with the fiducial $n=3$ value. This value is ruled out with
99.95\% confidence for GRB 140903A. We discuss possible causes of $n<3$ braking
indices in millisecond magnetars, showing that several models can account for
the measurement of the braking index in GRB 140903A, while it is more difficult
to account for a braking index consistent with $n=3$.
