The type Ia supernova (SN Ia) SN 2020nlb was discovered in the Virgo
Cluster galaxy M85 shortly after explosion. Here we present observations
that include one of the earliest high-quality spectra and some of the
earliest multi-colour photometry of a SN Ia to date. We calculated that
SN 2020nlb faded 1.28+/-0.02 mag in the B band in the first 15d after
maximum brightness. We independently fitted a power-law rise to the
early flux in each filter, and found that the optical filters all give a
consistent first light date estimate. In contrast to the earliest
spectra of SN 2011fe, those of SN 2020nlb show strong absorption
features from singly ionised metals, including Fe II and Ti II,
indicating lower-excitation ejecta at the earliest times. The spectra of
SN 2020nlb then evolve to become hotter and more similar to SN 2011fe as
it brightens towards peak. We also obtained a sequence of nebular
spectra that extend up to 594 days after maximum light, a phase out to
which SNe Ia are rarely followed. The [Fe III]/[Fe II] flux ratio (as
measured from emission lines in the optical spectra) begins to fall
around 300 days after peak; by the +594d spectrum, the ionisation
balance of the emitting region of the ejecta has shifted dramatically,
with [Fe III] by then being completely absent. The final spectrum is
almost identical to SN 2011fe at a similar epoch, and in sharp contrast
to a late nebular spectrum of SN 1994D, which still displayed strong [Fe
III] emission at these late times. Comparing our data to other SN Ia
nebular spectra, there is a possible trend where SNe that were more
luminous at peak tend to have a higher [Fe III]/[Fe II] flux ratio in
the nebular phase, but there are also notable outliers. Finally, using
light-curve fitting on our data, we estimate the distance modulus for
M85 to be 30.99+/-0.19 mag, corresponding to a distance of
15.8^{+1.4}_{-1.3} Mpc.