Albert Barnes Commentary Psalms 136:3

"Oh give thanks unto the Lord of lords; For his lovingkindness [endureth] for ever:" — Psalms 136:3 (ASV)

O give thanks to the Lord of lords - The Lord or Ruler of all in authority—all kings, princes, rulers. He is supreme over all. This is an attribute of Divinity; yet this is ascribed to the Lord Jesus, thus proving that he is divine. See the notes at (Revelation 19:16).

For his mercy ... - The ground of praise here, as in the previous verse, is that this God—the Supreme Ruler over all the potentates and magistrates of earth—is a merciful Being. He is kind and benevolent toward those rulers, and through them to mankind.