  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 15 d 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. These earliest
spectra show some similarities to maximum-light spectra of 1991bg-like SNe Ia.
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 +594 d 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.
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 is a notable outlier in SN
2003hv. 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.
