Why do Muslims read Surah al-Mulk at night?

The Prophet's ﷺ bedtime habit

Jabir reported that the Prophet ﷺ would not go to sleep until he had recited Surah as-Sajdah (chapter 32) and Surah al-Mulk (chapter 67) (Jami' at-Tirmidhi 3404). So reading al-Mulk before bed is simply following his example, one of many small sunnahs of the night. Others include the three Quls and Ayat al-Kursi. See What should I say before sleeping?

A surah that intercedes

The Prophet ﷺ said that a surah of the Quran with thirty verses interceded for a man until he was forgiven, and that it is "Blessed is He in whose hand is dominion" (Jami' at-Tirmidhi 2891). That is the opening line of al-Mulk:

Companions such as Abdullah ibn Mas'ud also described al-Mulk as "the protector" that shields its reciter from the punishment of the grave, and many scholars teach it with that hope. To learn more about that stage, see What happens in the grave in Islam?

What the surah talks about

Al-Mulk ("The Dominion" or "The Sovereignty") is a Makkan surah of 30 verses. It reminds us that Allah created life and death to test who is best in deeds, invites us to look at the sky and find no flaw in it, describes the regret of people who ignored the warning, and ends by asking who could bring us water if it sank into the earth. It is a perfect reflection before sleep, which the Quran describes as Allah taking the soul for a while (Quran 39:42).

How to make it a habit

• Any time after Maghrib counts as "night"; you don't have to read it in bed, though you can. See Can I read the Quran in bed or lying down?
• Read from a mushaf, an app or memory. Many converts start with the Arabic plus a translation, or listen while following along.
• Memorize it slowly. Two or three verses a week and you'll know it within a few months.
• Don't stress if you miss a night. It's a sunnah, not a duty. Pick it up again the next night.