"Alecto, Amycus’s sister, teaches Muggle Studies, which is compulsory for everyone. We’ve all got to listen to her explain how Muggles are like animals, stupid and dirty, and how they drive wizards into hiding by being vicious toward them, and how the natural order is being re-established. I got this one for asking her how much Muggle blood she and her brother have got." —Neville Longbottom on Alecto's teaching
Professor Alecto Carrow was a witch and sister of Amycus Carrow, a sadistic woman, and a Death Eater. She fought at the First Wizarding War, along with her brother. Alecto and Amycus didn't search for Lord Voldemort after his first downfall, but it was forgiven after Voldemort's return in 1995. Alecto also fought at several battles of the Second Wizarding War, includind the Battle of the Astronomy Tower. She became professor of Muggle Studies in 1997, and taught the students that wizards shouldn't associate with muggles. She also tortured the students who opposed them. She was eventually stunned by Luna Lovegood in the Ravenclaw Tower, and tied up by Professor McGonagall before the Battle of Hogwarts. After the battle, she was likely sent to Azkaban.
First Wizardry War
Alecto and her brother were Death Eaters during the First Wizarding War. It is possbile that they fought in several battles of the First War. After Lord Voldemort's downfall, Alecto and Amycus were among those who didn't search for Lord Voldemort, believing that that he was gone forever. They also managed to stay out of Azkaban.
Second Wizardry War
Alecto and Amycus most likely participated in Voldemort's return, and started to serve him again. During the war, Alecto was suspected to be a Death Eater, as she had associated with the Dark Lord. She was featured on one of the Ministry´s Wanted posters, along with her brother, Bellatrix Lestrange and Fenrir Greyback.
Battle of the Astronomy Tower
Albus Dumbledore: "Good evening Amycus. And you've brought Alecto too... charming..." Alecto Carrow: "Think your jokes'll help keep you on your death bed, then?" Albus Dumbledore: "Jokes? No, no, these are manners" — Albus Dumbledore and Alecto Carrow, after the former is 'cornered' at the Astronomy Tower.
Alecto and Amycus were among the group of Death Eaters who led an assault at Hogwarts School of Witchcraft and Wizardry in 1996, which ended with the death of Albus Dumbledore.
Appointment at Hogwarts
In 1997, after control of the Ministry of Magic fell to the Death Eaters, Alecto was made the new Muggle Studies Professor at Hogwarts School of Witchcraft and Wizardry after the murder of Charity Burbage. Muggle Studies became a compulsory course for all students, and Alecto changed the curriculum so instead of teaching equality and how Muggle technology worked, it became a brainwashing lesson of how Muggles are worthless and disgusting and should be treated like animals. When Neville Longbottom challenged her on this, Alecto slashed him across the face. She and her brother were also the ones that handed out punishment to all students during their reign at Hogwarts, and beat and tortured students who defied them.
Battle of Hogwarts
Harry: "Wit beyond measure is man's greatest treasure." Alecto: "That makes you pretty skint, witless." — Alecto mocking Harry Potter
In 1998, on Lord Voldemort's orders, Alecto guarded the Ravenclaw common room, where Harry Potter might show up on his search for the Horcruxes. She caught Harry and called the Dark Lord using her Dark Mark, but was stunned by Luna Lovegood, and later was tied up along with her brother, Amycus Carrow, by Minerva McGonagall.
After the Battle of Hogwarts and Voldemort's final defeat, Alecto and her brother were presumably sent to Azkaban or killed.