Is a Marriage Contract Valid Without the Guardian?

It is not valid. There is no contract except with a guardian. The Prophet ﷺ says in the authentic hadith:

لَا نِكَاحَ إِلَّا بِوَلِيٍّ “There is no marriage except with a guardian.”

So the woman does not marry herself off, nor does she marry off another. There must be a guardian in the marriage.

The marriage is attended by four: the guardian, the husband, and the two witnesses. This is the legislated marriage. The guardian is the closest male relative to the woman: her father, then her grandfather however high among the males, then her son, then her son’s son however low, then her full brother, then her half-brother through the father, then the rest of the male relatives, the nearest and then the next nearest. There must be a guardian, and he should be a Muslim, known for goodness, and upright according to what is possible. If an upright one is not available, it is permitted even if he is not upright, so long as he is a Muslim.

The point is that there must be a guardian, and there must be two witnesses known for goodness and integrity. The husband is the one seeking the marriage, and the guardian conducts it and says, “I marry you to so-and-so, daughter of so-and-so,” or his daughter, or his sister, or the like. The husband says, “I accept,” and the two witnesses bear witness to that.

As for what is called the “customary marriage” (zawāj ʿurfī), which is that she marries herself off without a guardian, this is not permitted; rather it is invalid according to the majority of the scholars. The authentic text from the Messenger of Allah ﷺ points to this, his saying, “There is no marriage except with a guardian,” and his saying ﷺ:

لَا تُزَوِّجُ الْمَرْأَةُ الْمَرْأَةَ، وَلَا تُزَوِّجُ الْمَرْأَةُ نَفْسَهَا “A woman may not marry off a woman, and a woman may not marry off herself.”

Source: binbaz.org.sa (Nūr ʿalā al-Darb).