Where the king's prisoners were bound Potiphar, it is likely, chose that prison because it was the worst; for there “the irons entered into the soul,” Psalms 105:18, but God designed it to pave the way to his enlargement. Our Lord Jesus, like Joseph, was bound, and numbered with transgressors. But the Lord was with Joseph, and showed him mercy. No gates nor bars can shut out his gracious presence from his people. God gave him favour in sight of the keeper of the prison God can raise up friends for his people, even where they little expect them. The keeper saw that God was with him, and that every thing prospered under his hand, and therefore intrusted him with the management of the affairs of the prison.
Related Commentaries of Genesis 39:20
Genesis 39:20-21
20 And Joseph's master took him, and put him into the prison, a place where the king's prisoners were bound: and he was there in the prison.
21 But the LORD was with Joseph, and shewedb him mercy, and gave him favour in the sight of the keeper of the prison.