The first time Harry successfully cast a Patronus was during that Quidditch match against Ravenclaw. After weeks of hard work with a Boggart-Dementor that forced him to relive his mother’s death, this incredibly advanced spell suddenly came surprisingly easily to Harry – which was obviously because, as Lupin later informed him, it was not Dementors he was fighting, but Malfoy and his cronies:
As Patronus-casting exercises go it’s not life-saving, but it did help Harry’s Quidditch team secure an epic victory. We think that counts.
Harry’s actual first Patronus was more impressive, even if it did initially cause some confusion – Harry saw his future self standing on the opposite bank; briefly thought he was some unlikely reincarnation of his own Dad; nearly suffered the Dementor’s kiss alongside Hermione and Sirius… well, it was all a bit of a muddle.
That is, until Harry-from-the-future (who, it turns out, really does look like his Dad) saved them all by successfully casting an actual Patronus, which scattered Dementors like skittles before ending with a bow.
As Dumbledore later told Harry: “‘Prongs rode again last night.’” So that’s that cleared up.
By Harry’s fifth year he was a dab hand at the old Patronus charm, having also used it to charge a Boggart during the final Triwizard Tournament task. But nothing prepared him for the appearance of Dementors near the Dursley’s home in Little Whinging, the summer before he returned to Hogwarts. And yet, there they were, ready to leech life out of any human who crossed their path — even if that human was Dudley Dursley.
Handy for Dudley, then, that Harry was such a dab hand with the old Patronus charm:
Rescuing Dudley’s life also led to a thawing in relations between Harry and his cousin. Not that Harry realised at the time, but what greater way to show thanks than a nice cup of tea? Even if said tea was cold, left on the landing, and delivered by Dursley two years later. It’s still better than nothing.
Patronuses weren’t just useful for defeating Dementors – in a world where there was (apparently) no such thing as encrypted texts, they also provided a foolproof method of communication. Tonks used hers to let Dumbledore know Harry had been safely located on the Hogwarts Express, following his run-in with Malfoy. Kingsley Shacklebolt used his to let everyone at Bill and Fleur’s wedding know the Ministry had fallen. And during the Battle of Hogwarts, Professor McGonagall used hers to call the rest of the Heads of House to Harry’s aid.
Because who wouldn’t want to receive a personalised message delivered by a shining silver animal? Much more attractive than a DM, if you ask us. Not to mention a lot more reliable.
When Harry, Ron and Hermione entered the Ministry of Magic in their Polyjuice-Potion-enabled disguise, it was good news for those summoned to stand before the Muggle-born Registration Commission. Even disguised as a thoroughly mean wizard, Harry couldn’t stop himself from rescuing others:
And for Mary Cattermole and the other Muggle-borns about to plead their case, Harry’s Patronus was a life-saver. Along with Hermione’s (slightly less corporeal) Otter Patronus, Prongs led the Muggle-borns out of the dungeon. Which was perhaps not the most discreet way to escape but, as always, you can’t fault Harry’s commitment to the rescue attempt.
But of course, it was not just Harry whose Patronus saved the day.
With Harry and Hermione despondent after the events in Godric’s Hollow and Ron trying desperately to find his way back to them, it was Snape’s Patronus – the Silver Doe that was somehow familiar to Harry, perhaps because it recalled his mother’s own Patronus – that brought them back together.
It also led Harry to the Sword of Gryffindor, without which they may not have been able to destroy the Horcrux-locket at all. So, pretty handy all round.