  Metal-poor globular clusters (GCs) are both numerous and ancient, which
indicates that they may be important contributors to ionizing radiation in the
reionization era. Starting from the observed number density and stellar mass
function of old GCs at $z=0$, I compute the contribution of GCs to ultraviolet
luminosity functions (UVLFs) in the high-redshift Universe ($10 \gtrsim z
\gtrsim 4$). Even under absolutely minimal assumptions - no disruption of GCs
and no reduction in GC stellar mass from early times to the present - GC star
formation contributes non-negligibly to the UVLF at luminosities that are
accessible to the Hubble Space Telescope (HST; $M_{1500} \approx -17$). If the
stellar masses of GCs were significantly higher in the past, as is predicted by
most models explaining GC chemical anomalies, then GCs dominate the UV emission
from many galaxies in existing deep-field observations. On the other hand, it
is difficult to reconcile observed UVLFS with models requiring stellar masses
at birth that exceed present-day stellar masses by more than a factor of 10.
The James Webb Space Telescope will be able to directly detect individual GCs
at $z \sim 6$ in essentially all bright galaxies, and many galaxies below the
knee of the UVLF, for most of the scenarios considered here. The properties of
a subset of high-$z$ galaxies with $-19 \lesssim M_{1500} \lesssim -14$ in HST
lensing fields indicate that they may actually be GCs in formation.
