  The precise localization (<1'') of multiple fast radio bursts (FRBs) to z>0.1
galaxies has confirmed that the dispersion measures (DMs) of these enigmatic
sources afford a new opportunity to probe the diffuse ionized gas around and in
between galaxies. In this manuscript, we examine the signatures of gas in dark
matter halos (aka halo gas) on DM observations in current and forthcoming FRB
surveys. Combining constraints from observations of the high velocity clouds,
OVII absorption, and the DM to the Large Magellanic Cloud with hydrostatic
models of halo gas, we estimate that our Galactic halo will contribute ${\rm
DM}_{\rm MW,halo} \approx 50-80 \rm pc/cm^{-3}$ from the Sun to 200 kpc
independent of any contribution from the Galactic ISM. Extending analysis to
the Local Group, we demonstrate that M31's halo will be easily detected by
high-sample FRB surveys (e.g. CHIME) although signatures from a putative Local
Group medium may compete. We then review current empirical constraints on halo
gas in distant galaxies and discuss the implications for their DM
contributions. We further examine the DM probability distribution function of a
population of FRBs at z >> 0 using an updated halo mass function and new models
for the halo density profile. Lastly, we illustrate the potential of FRB
experiments for resolving the baryonic fraction of halos by analyzing simulated
sightlines through the CASBaH survey. All of the code and data products of our
analysis are available at https://github.com/FRBs.
