Minerva McGonagall (born 4 October c. 1925) was the Transfiguration teacher at Hogwarts from 1956 to 1998[HP5], and also served as Head of Gryffindor House and Deputy Headmistress. After the defeat of Voldemort she became Headmistress of Hogwarts.
Minerva McGonagall became Acting Headmistress of Hogwarts School of Witchcraft and Wizardry on the death of her esteemed colleague Albus Dumbledore upon his death and other the tragic events of Harry Potter's sixth year. After the Death Eaters took control of the Ministry of Magic, Severus Snape became Headmaster, but McGonagall is believed to have taken over on his death. It is outside the scope of the Harry Potter story to know the identities of her successors as Transfiguration teacher and Head of Gryffindor House.
She is famous for three things: firstly, she was a very strict, no-nonsense teacher of Transfiguration, held in respect (and some fear) by all students; secondly, she is an avid Quidditch fanatic; and thirdly, she is one of the few Animagi of the century, her Animagus form being a tabby cat with markings near her eyes resembling her square spectacles. These first two facets demonstrate the conflicting sides of her personality: on the one hand, she is strict, severe and unsmiling, while on the other, she longs for her house to be victorious in a trivial sporting event. Although, it must be noted here, that after Albus Dumbledore's recent death, Quidditch will not likely continue to be one of her top priorities. That is, at least until things resume some semblance of normalcy.
Minerva sees other forms of magic less elegant and more simplistic then the art of Transfiguration. She is infamously known for giving vast amounts of homework. This thinking generally applies to Divination, a subject McGonagall dislikes greatly, and seems to think low of the Divination Professor Sibyll Trelawney. She shares a lot in common with the character Hermione Granger, whose descriptions cross frequently with each other. There have been a few incidents when Minerva showed her softer side. An example is when she gave permission for Harry and Ron to visit Hermione in the Hospital Wing when she was petrified. She had a tear in her eye when she said this. She also giggled and blushed when a tipsy Hagrid kissed her.
No one could have ever been in any doubt as to who was in charge when Professor McGonagall was teaching a class – even Dolores Umbridge was treated with very little respect (and a good deal of contempt) when attempting to assert her authority in McGonagall's classroom. She tolerates no misbehaviour and commanded the attention of the whole of her class. Unlike Professor Severus Snape, she is also careful to take an even-handed approach to discipline with all pupils, punishing those from her house even when this could mean losing the House Cup.
Upon Albus Dumbledore's death in 1997, McGonagall became Headmistress of Hogwarts. However, subsequent to Voldemort's takeover of the Ministry of Magic, Severus Snape was appointed Headmaster and McGonagall returned to teaching Transfiguration and leading Gryffindor House. (Voldemort, believing Snape to be loyal to him, wanted Snape as headmaster so that he could exercise control over magical education.)
Professor McGonagall played a pivotal role in the final battle of the series, the Battle of Hogwarts in Harry Potter and the Deathly Hallows. With help from Harry Potter and Luna Lovegood, she incapacitated the Death Eaters Amycus and Alecto Carrow and drove Severus Snape out of the castle; she then led the teachers and other defenders of Hogwarts in battle against the Death Eaters. Although Snape's true loyalty was revealed to be to Dumbledore, he is killed by Voldemort, and it is implied that McGonagall resumes her rightful role as Headmistress.
Professor McGonagall was the Head of Gryffindor house. In that capacity, it is unlikely that she would have taken her usual unbiased approach to discipline as far as to harm her beloved Quidditch team. Indeed, when she caught Harry disobeying Madam Hooch's instructions and flying a broomstick having been told not to in his first year (albeit in an attempt to retrieve Neville's Remembrall from Malfoy), her reaction was to appoint him Seeker of the Gryffindor Quidditch team rather than to punish him. However, she does support Snape in giving Harry detention on the day of the final Quidditch match in Harry Potter and the Half-Blood Prince, after Harry attacks Draco Malfoy with the Sectumsempra spell.
This one small distraction aside, however, Minerva McGonagall is strict, straight and fair at all times. She was also fiercely supportive of Professor Dumbledore's reign as Headmaster, and being Deputy Headmistress, is his natural successor as Head of Hogwarts.
In an interview after the publication of Harry Potter and the Deathly Hallows, J. K. Rowling said that Hogwarts "now" (presumably meaning as of 2007) had a different Head,saying that McGonagall was "getting on a bit". So we can assume that McGonagall retired some years after the events of the final book and will not be Head in the time of the children featured in the Epilogue. (Dumbledore, however, became Headmaster at an age greater than that at which McGonagall retired).