Discovered most probably by Charles Messier in 1781. Independently rediscovered by William Herschel in the on April 8, 1784.

On March 18, 1781, Charles Messier discovered the unusually large number of 8 nebulous objects, all in the region of today's border of the constellations Virgo and Coma Berenices, i.e. Virgo Cluster galaxies (plus one additional object, globular cluster M92 in Hercules). The last of these eight objects was cataloged by him as M91, but his position was erroneous.

Thus, for a long time, M91 was a missing Messier object, as Messier had determined its position from M89 while he thought it was from M58, as the Texas amateur William C. Williams of Fort Worth has figured out in 1969 (Williams 1969). Thus, the identity of M91 with NGC 4548, which had been cataloged H II.120 by William Herschel on April 8, 1784, was finally uncovered. Previous opinions have been that M91 had either been a comet which the great comet hunter Messier mistook for a nebula, and Owen Gingerich had suspected that it had been a duplicate observation of M58. William Herschel had not found M91 at Messier's erroneous position and suspected that it might have been NGC 4571 (his H III.602), a beautiful but faint 11.3 mag barred spiral (NGC 4571 came into discussion in summer 1994 when a group of astronomers at the Canada France Hawaii Telescope (CFHT) used observations of 3 Cepheids in this galaxy for a determination of the Hubble constant).

The barred spiral galaxy M91 is an appealing member of the Virgo Cluster of Galaxies. It is of type SBb and its bar is very conspicuous, lying at position angle 65/245 degrees (as measured from the North direction to the East). As its recession velocity is only about 400 km/sec, it has a considerable peculiar velocity toward us through the Virgo cluster, about 700 km/sec, as the cluster's recession velocity is about 1100 km/sec.

The membership of Messier 91 in the Virgo Cluster of Galaxies was confirmed by a recent measurement of its distance as 52 +/- 6 million light years by detecting Cepheid variables. These measurements were done by the Hubble Space Telescope H0 Key Project Team (paper XX, 1997). This coincides well with the value for other Virgo galaxies, including M100. The difference of their to our value is mainly due to another distance of the Large Magellanic Cloud assumed by them - the Hipparcos correction would increase their distance to about 58 Mly.

For the moderately equipped amateur, M91 is one of the most difficult Messier objects. Suggestions of this bar may be seen at medium power even in smaller telescopes, if the viewing conditions are good enough to see the galaxy at all. Photos show the bar more clearly, and show the spiral arms emanating from the ends of the bar.

Messier had described M91 as "Nebula without stars, fainter than M90". John Herschel described it as bright, large, little elongated, little brighter in the middle in his General Catalogue, while in his earlier observations, he describes its shape once as "pretty much elongated" and twice as "round". This is probably because of different viewing conditions: Under poor conditions, only the bright elongated bar region of this galaxy shows up, while under good conditions the spiral arms show up and exhibit an almost round to slightly elongated shape. This effect can be reproduced to some degree with amateur instruments.