Albert Barnes Commentary Psalms 74:12

"Yet God is my King of old, Working salvation in the midst of the earth." — Psalms 74:12 (ASV)

For God is my King of old - That is, the king, or ruler of his people. The people had acknowledged him as their king and ruler, and he had shown himself to be such. This is given as a reason why he should now intervene on their behalf. It is an argument, always proper to urge, drawn from the faithfulness and unchangeableness of God.

Working salvation in the midst of the earth - Salvation for his people. The reference here particularly is to what he had done for his people in delivering them from bondage in Egypt, and conducting them to the promised land, as is stated in the following verses.