The Killarney-born rider looked set for a comfortable defence of his title a few weeks ago, but a late charge from William Buick set up a thrilling finale.
Buick closed the gap to just one on a couple of occasions earlier in the week, but a double at Haydock on Friday meant Murphy started the final day with a decisive three-winner lead.
Buick struck early on Champions Day with Creative Force in the big sprint to give himself renewed hope - but defeat for Adayar in the Champion Stakes put paid to his gallant season-long pursuit of Murphy.
Murphy said: "I'm over the moon and thrilled to get it across the line. Thank you to my family and everyone - I've got a massive support group.
"The last week I've had a lot of people on my side. Frankie Dettori has been there every day making sure my spirits are up and I was riding off instinct as normal which is key.
"You need to make every ride count and I was running out of opportunities. I held it together thankfully in the end."
However, it has not been entirely plain sailing for the 26-year-old, who just last week was forced to give up rides at Newmarket after failing a racecourse breathalyser test amid reports of an incident in a pub the previous night.
Murphy is also still sporting the scars of a nasty face injury suffered in a parade ring fall Salisbury last month.
Murphy added: "I think everyone knows that I am human and quite honest. But I need to do better and I don't want any issues surrounding my career. Let's just focus on riding winners and hopefully winning another jockeys' championship.
"I'm 26-years-old so I think I will keep trying for a few more years.
"Horses are my life and I'm never happier than when I'm on the back of a horse. I've bought my showjumpers and they keep me busy too.
"Sheikh Fahad and Qatar Racing have a few horses to go to America for the Breeders' Cup. There are lots of international races in Hong Kong and Japan coming up so I will be busy.
"William was getting a lot more support than I was in the last couple of weeks and the likes of Ed Walker, Hughie Morrison, Andrew Balding and Saeed bin Suroor have been amazing so thanks to them."
His boss Sheikh Fahad of Qatar Racing remains fiercely loyal to Murphy, as does his racing manager David Redvers, although he admitted there are things the champion "will work hard to put right".
He told Racing TV: "Oisin has really felt it this time, there's no question about it. He felt it at the end of last season as well when he was over at the Breeders' Cup and William was eating into his lead in dramatic fashion.
"It's been a really high pressure and tense end to the season. Thank God it ends today and not at Doncaster in a few weeks' time as that would drag it out even more.
"Oisin is a young man. We've known him since he was 17 and he's still growing and developing as a man.
"Clearly there are things that have happened in recent times that he has regrets about and will work hard to put right.
"I have no doubt that when the pressure of Champions Day is out of the way and he can breathe and regroup, that there will be changes made to his lifestyle and he's going to look very hard at that.
"Nobody can have any idea what it's like being in this cauldron unless they've actually been there themselves.
"I spoke to Kevin Darley the other day and he said when he sat down at the end of his championship-winning season, he literally lost two days of his life he was so exhausted. He couldn't even remember what happened in that time.
"William is going to be exhausted as well. Oisin is exhausted and puts himself under huge pressure.
"We all make mistakes. It's been difficult and obviously it's disappointing for everybody that the stories in the press have detracted from what is an absolutely unbelievable achievement - to win three championships on the bounce from such an early start.
"We're incredibly proud of him. He's very much part of our family and whatever help Oisin needs he'll get."
On his bid for a first tile, Buick said: "I gave it everything I had, I left nothing on the table and I didn't quite make it.
"I had a lot great support all the way through, these last few days the support and well wishes I've had has been really overwhelming and I'm really grateful for that.
"My family and friends have been behind me the whole way, it's a tough thing to do and you've got to be disciplined, you've got to work really hard.
"I've had some great support from trainers and owners all around and it's a big 'thank you' to them because without the support it's not possible.
"As hard work as it is, I've enjoyed it as well and I feel like it's brought me on and now there's still a lot of good races around the world before the end of the season.
"I'll get the winter out of the way and re-group again for next year."
His boss Charlie Appleby said: "He changed agents last year, Tony Hind has been sending him all around the country and I think it's something William himself will say he's enjoyed
"It's all very well riding these good horses at the big meetings, but he's been mucking out at the smaller tracks and success breeds success - you can't beat having winners.
"William riding more has given him more opportunity to ride more winners and we've seen a better rider as a result.
"He's always been a rider at the top of his game, but there's no doubt that now he's riding so many different horses his style of riding is changing and he's hungry.
"Win, lose or draw, he's not going to walk away deflated after what he's achieved this year. He's got that character where he'll go forward again next year and do the same, I'm sure.
"For me he's a huge part of the team and the more winners he rides the happier I am!"