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The type Ia supernova (SN Ia) SN 2020nlb was discovered in the Virgo Clustergalaxy M85 shortly after explosion. Here we present observations that includeone of the earliest high-quality spectra and some of the earliest multi-colourphotometry of a SN Ia to date. We calculated that SN 2020nlb faded 1.28+/-0.02mag in the B band in the first 15d after maximum brightness. We independentlyfitted a power-law rise to the early flux in each filter, and found that theoptical filters all give a consistent first light date estimate. In contrast tothe earliest spectra of SN 2011fe, those of SN 2020nlb show strong absorptionfeatures from singly ionised metals, including Fe II and Ti II, indicatinglower-excitation ejecta at the earliest times. The spectra of SN 2020nlb thenevolve to become hotter and more similar to SN 2011fe as it brightens towardspeak. We also obtained a sequence of nebular spectra that extend up to 594 daysafter maximum light, a phase out to which SNe Ia are rarely followed. The [FeIII]/[Fe II] flux ratio (as measured from emission lines in the opticalspectra) begins to fall around 300 days after peak; by the +594d spectrum, theionisation balance of the emitting region of the ejecta has shifteddramatically, with [Fe III] by then being completely absent. The final spectrumis almost identical to SN 2011fe at a similar epoch, and in sharp contrast to alate nebular spectrum of SN 1994D, which still displayed strong [Fe III]emission at these late times. Comparing our data to other SN Ia nebularspectra, there is a possible trend where SNe that were more luminous at peaktend to have a higher [Fe III]/[Fe II] flux ratio in the nebular phase, butthere are also notable outliers. Finally, using light-curve fitting on ourdata, 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.

Observations of type Ia supernova SN 2020nlb up to 600 days after explosion, and the distance to M85