William Herschel discovered NGC 5820 = H II-756 = h1898 on 5 May 1788 (sweep 842) and recorded "cF, pL, iF, r." His position is poor - about 7' southwest of UGC 9642. JH called this object "B; R; sbM; precedes a splendid double star." His position is just 1' north of center. Because of his father's poor position, he assigned a separate GC designation to his object, but they were combined in the NGC.

Samuel Hunter observed NGC 5820 and 5821 on 29 Apr 1861 with the 72" and wrote, "[NGC 5820] is vB, E pf and has Nucl, about 2' ssf [of NGC 5820] is an eeF, S neb., 3' f [NGC 5820] is a F neb or possibly a D * [it is a double star]; 3 nf is a F, pL neb [NGC 5821], vgbM to a dull nucl." At 1.7' SSE of NGC 5820 is SBS1457+540 = PGC 140436 at V = 15.7. Although this galaxy is quite faint, it matches Hunter's position and within the magnitude range of the 72". Dreyer added a note later that "the eeF neb would appear to be a nova", but perhaps because it wasn't confirmed on another observation he didn't include it in the NGC.

400/500mm - 17.5" (5/23/87): moderately bright, small, elongated 2:1 E-W, bright core. Forms a pair with NGC 5821 3.7' NE. Located 8' W of the wide double star ADS 9474 = 6.8/7.4 at 40".

Notes by Steve Gottlieb