Though his name is missing from the Sumerian king list, Mesilim is among the earliest historical figures recorded in archaeological documents.

[5] 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 Ištaran, Mesilim established a new border between Lagash and Umma, and erected a pillar to mark it, on which he wrote his final decision.

[7] 8–12 𒈨𒁲 𒈗𒆧𒆠𒆤 𒅗 𒀭𒅗𒁲𒈾𒋫 𒂠 𒃷 𒁉𒊏 𒆠𒁀 𒈾 𒉈𒆕 me-silim lugal kiški-ke4 inim dištaran-na-ta eš2 gana2 be2-ra ki-ba na bi2-ru2 "Mesilim, king of Kiš, at the command of Ištaran, measured the field and set up a stele there."

In Sumerian version, the proverb reads "The E-babbar which Mesilim had built, Annane, the man whose seed was cut off, tore down."