Inside the pyramid of King Unas, texts were written to tell the priests how to say the funeral ritual. These "pyramid texts" are important parts of ancient Egyptian religion, and they're the first place we see Ma'at's name in religious writing.
In Dynasty 5, the vizier (pharaoh's personal assistant, sort of like a vice president) was called the Priest of Ma'at and was the chief justice of Egypt's government. Later on, all judges were called priests of Ma'at, and her symbols, the feather and the straightedge (like a ruler, used in architecture) were used by judges on their clothes and personal seals, and even decorated their tombs.
New Kingdom pharaohs had their tombs hidden in a valley on the western side of the city of Thebes (modern day Luxor). It's called the Valley of the Kings today, but ancient Egyptians called it "The Place of Ma'at" or "The Place of Truth."
In the afterlife, you have to go before 42 judges—one for every nome (or state) of ancient Egypt—and tell them all the things you did not do wrong. Chapter 125 in the Book of Going Forth by Day (Peret em-Heru), the Ancient Egyptian Book of the Dead, lists the 42 "negative confessions." They're sort of like the Ten Commandments, except there are more of them, and they're in the past tense: "I did not kill" instead of "Thou shalt not kill," for example. These scary judges, all fierce gods and spirits who can tell if you're lying, are called the Assessors of Ma'at.
Ma'at appears in the judgment scene of the Book of the Dead. Right after the dead person passes the 42 Assessors, she offers her feather of truth to be weighed against his or her heart, to decide if it's good enough. If it is, the person gets to go to the kingdom of Osiris. If a person's heart does not weigh the same as Ma'at's feather—if it's too heavy or too light—the heart is fed to Ammit, "swallower of the dead." In case you're worried you can't pass Chapter 125, there's hope. Chapter 126 offers magic words you can say to convince your heart not to betray you.
Amunhotep III built a temple to Ma'at, the earliest one we know about, inside the precinct of Amun-Ra's giant temple at Karnak, in modern-day Luxor, Egypt. Later on, a handful of chapels and temples for Ma'at were also made in other cities.
The pharaoh Akhenaten (who was really Amunhotep IV, Amunhotep III's second son) starts using the phrase "living in Ma'at" (ankh em Ma'at) in relation to his new, sun-based religion. Unfortunately for Akhenaten, Egypt's priesthood didn't believe he really was living in Ma'at, so his new religion ended with his own death.
At Abu Simbel, Ramses II built two temples: a huge one for himself and his gods, and a smaller, prettier one for his wife and the goddess Hathor. Over the doorway of the bigger temple is a giant carving of Ra-Horakhty, with a was-scepter under one hand and a seated Ma'at under the other. This isn't just a strange image: it's a hieroglyph rebus, spelling out Ramses II's throne name: User-Ma'at-Ra, or "Ra's Ma'at is Strong." Very clever.
This statue shows vizier Psamtikseneb, a priest of Ma'at, wears the Lady of Justice on a pendant around his neck to show he is a judge, while carrying a tiny shrine with the god Mnevis inside. This kind of tiny shrine was common in the Late Period—good for the priests, since bigger stone statues get heavy after a while!