  The fluid forces associated with primordial magnetic fields (PMFs) generate
small-scale fluctuations in the primordial density field, which add to the
$\mathrm{\Lambda CDM}$ linear matter power spectrum on small scales. These
enhanced small-scale fluctuations lead to earlier formation of galactic halos
and stars and thus affect cosmic reionization. We study the consequences of
these effects on 21 cm observables using the semi-numerical code 21cmFAST
v3.1.3. We find the excess small-scale structure generates strong stellar
radiation backgrounds in the early Universe, resulting in altered 21 cm global
signals and power spectra commensurate with earlier reionization. We restrict
the allowed PMF models using the CMB optical depth to reionization. Lastly, we
probe parameter degeneracies and forecast experimental sensitivities with an
information matrix analysis subject to the CMB optical depth bound. Our
forecasts show that interferometers like HERA are sensitive to PMFs of order
$\sim \mathrm{pG}$, nearly an order of magnitude stronger than existing and
next-generation experiments.
