Alecto Carrow was born sometime before 1981 into the pure-blood Carrow family, somewhere in the British Isles. At some point before her instruction in the magical arts, she acquired a wand.
Given that she seemed to have been acquainted with Albus Dumbledore, it is possible she attended Hogwarts School of Witchcraft and Wizardry in her youth.
First Wizarding WarEdit
Alecto and her brother Amycus supported Lord Voldemort and became Death Eaters during the First Wizarding War. After Voldemort's downfall in 1981, they did not search for their master, believing that Voldemort was gone forever and they managed to avoid imprisonment in Azkaban.
Despite all this, the Carrows were among those who, after Voldemort's return, resumed their Death Eater activities. The two siblings were fairly active during the Second Wizarding War. Like Bellatrix Lestrange, Alecto admired Voldemort and detested blood traitors.
Second Wizarding WarEdit
Rebirth of the Dark LordEdit
Alecto and Amycus were, most likely, among the Death Eaters that were summoned by Lord Voldemort to the churchyard at Little Hangleton on June 24, 1995, as their absence wasn't noted by Voldemort himself.
Battle of the Astronomy TowerEdit
In the evening of June 30, 1997, Alecto and her brother participated in the assault of Hogwarts Castle, orchestrated by Draco Malfoy, that resulted in the death of Albus Dumbledore. When the Death Eaters arrived at the top of the Astronomy Tower, where Dumbledore was trapped, Alecto gave "wheezing giggles", as her brother made fun of the Headmaster. When Severus Snape arrived on the scene, he cast the curse that ended the Headmaster's life.
On Snape' sorders, she and her brother quickly left the Tower, panting excitedly. In the corridors below, Alecto and Amycus retreated after Snape with their arms over their heads, under attack from blasts by Minerva McGonagall. As the brother and sister descended the Marble Staircase, Harry Potter took a shortcut and arrived in the grounds before them.
As the two Death Eaters closed in after him, one of them cast a spell that sent Harry to the ground. Harry hit one of them, who stumbled and fell, tripping up the other, with another Impediment Jinx. The two siblings then ran to the gates and Disapparated beyond the school boundaries.
That summer, the Carrows attended a Death Eater meeting at Malfoy Manor concerning plans to capture or kill Harry Potter and take over the Ministry of Magic.
Appointment at HogwartsEdit
On August 1, 1997, the Death Eaters managed to stage a coup within the Ministry of Magic, thus gaining influence over all of wizarding Britain. A month later, on September 1, Amycus and Alecto were made Defense Against the Dark Arts teacher and Muggle Studies teacher, respectively, as well as Deputy Headmasters under newly appointed Headmaster Severus Snape.
The Muggle Studies classes were made compulsory core classes. The curriculum was changed so that it now covered the Ministry of Magic's anit-Muggle propaganda; that Muggles were stupid and dirty, how they threatened the Wizarding world and how the "natural order" was being re-established, instead of covering aspects of Muggle culture and science.
As Deputy Heads, the Carrows were put in charge of all punishment. The members of faculty were obliged to report any rule-breaking student to them, although they didn't if they could avoid it. Punishments given out by the Carrows included torture, via the Cruciatus Curse, as well as chaining students and corporal punishment in the form of beatings and slashes with knives.
Dumbledore's Army, led by Neville Longbottom, spearheaded the resistance against the Carrows. Alecto made a slash on his face after he asked her how much Muggle blood she and her brother had. The group, now made illicit after the re-enactment of the Educational Decree Number Twenty-Four, used to sneak into the corridors at night and put up graffiti on the walls with messages like "Dumbledore's Army, Still Recruiting". Michael Corner was caught releasing a first year the Carrows had chained up and was violently tortured.
Once the Carrows realized they had no hold over Neville, they planned to kill him or send him Azkaban. As the Carrows chased him, Neville chose the Room of Requirement as a hideout, and the rest of Dumbledore's Army soon followed in his footsteps and moved into the room as the Carrows tried to hunt them all down.
By the evening of May 1, 1998, news of Harry Potter, Ronald Weasley, and Hermione Granger's break-in of Gringotts Wizarding Bank reached the student body; Terry Boot was beaten up by the Carrows for yelling about it in the Great Hall over dinner.
That same day, as Lord Voldemort became aware that Harry Potter, Ronald Weasley, and Hermione Granger were hunting down and destroying his horcruxes, he instructed Amycus and Alecto that Harry might try to get into the Castle and Ravenclaw Tower. At the Carrows' urgent request, Professor Flitwick, Head of Ravenclaw House, let Alecto into the common room, where she stood guard.
Alecto was keeping watch when Harry arrived with Luna Lovegood. She immediately touched her Dark Mark, letting Voldemort and all the Death Eaters know that they had got him. Alecto was immediately stunned by Luna, but Amycus demanded that Minerva McGonagall let him into the Tower. Once inside, he saw his unconscious sister but no sign of Harry. as both he and Luna were hidden beneath Harry's Invisibility Cloak. Aware that they were going to feel Voldemort's wrath if they did not hand him Harry, Amycus planned to pass it off on the students, and when Professor McGonagall objected, calling Amycus and Alecto cowards in the process, he spat in her face. This caused Harry to use the Cruciatus Curse on him, which knocked him out. Minerva McGonagall used a spell to conjure a net, and the two siblings were tied up and hoisted into the air, where they were left dangling beneath the ceiling.
Neither Alecto nor her brother fought in the Battle of Hogwarts as both of them were kept tied-up at Ravenclaw Tower. It is likely that after the battle and the end of the Second Wizarding War they both were imprisoned in Azkaban for their crimes under Kingsley Shacklebolt's de-corrupted Ministry of Magic.