Though his name is missing from the Sumerian king list, Mesilim is among the earliest historical figures recorded in archaeological documents. He reigned some time in the "Early Dynastic III" period (c. 2500-2330 BC). Inscriptions from his reign state that he sponsored temple construction in both Adab and Lagash, where he apparently enjoyed some suzerainty.
Mesilim is best known for having acted as mediator in a conflict between Lugal-sha-engur, his ensi in Lagash, and the neighboring rival city state of Umma, regarding the rights to use an irrigation canal through the plain of Gu-Edin on the border between the two. After asking the opinion of the god Satarana, Mesilim established a new border between Lagash and Umma, and erected a pillar to mark it, on which he wrote his final decision. This solution was not to be permanent; a later king of Umma, Ush, destroyed the pillar in an act of defiance.
In the 1950s, Sumerologist Edmund Gordon reviewed the literary evidence and suggested a tentative theory that Mesilim and King Mesannepada of Ur, who later in his reign also assumed the title "King of Kish", were in fact one and the same. Both names are known elsewhere from a unique Mesopotamian proverb about the king whose temple was torn down. In Sumerian version, the proverb reads "The E-babbar which Mesilim had built, Annane, the man whose seed was cut off, tore down." E-babbar was the temple in Lagash, and Gordon took Annane to be a corruption of the name A-anne-pada, i.e. Mes-anne-pada's own son. The much later Akkadian proverb reads "The temple which Mesannepadda had built, Nanna, whose seed was picked off, tore down". However, Thorkild Jacobsen disputed this theory and reached the opposite conclusion, that Mesilim and Mesannepada were probably distinct, arguing that the Akkadian scribe did not recognise the name of Mesilim that was not on the kinglist, and simply substituted that of a name he knew from the list.