But creator Sally Wainwright has admitted both she and Lancashire had worried the drama would "bomb" while filming the first series as they weren't sure whether viewers would get it.
Wainwright, who also wrote Last Tango In Halifax and Gentleman Jack, spoke to Radio 4's Today programme on Monday morning, where she said she had not been sure how people would respond to the gritty series about West Yorkshire's drug problem and one police sergeant's personal campaign against murderer and rapist Tommy Lee Royce, played by James Norton.
Asked whether she had known Catherine would become so popular with audiences, she said: "No, I don't think you ever do when you start writing a show. You hope they will, and it's kind of gold dust when it happens.
"I remember one day when we were filming the first series, Sarah saying to me, 'God, I hope people get this,' and it's kind of inconceivable now that they wouldn't, but you never know when you make a show.
"You put the same amount of effort into every show you make and some shows bomb and others don't, and you never really know til they're out there and people are watching them and responding to them."
Viewers have been calling for Lancashire to win awards as the straight-talking Catherine, who manages to be tough on criminals and her own family while showing empathy for them and who gets some of the BBC One series' best lines.
But Wainwright said she was annoyed at suggestions that she always portrays her female and male characters in the same way.
She said of Catherine: "She's complicated. I get accused of writing strong women and weak men, and that annoys me a bit because I think a good character is a complicated character and I think she's complicated and she's not always very likeable."
Of show villain Tommy, she added: "I think he is weak actually, he's physically strong, but he's mentally weak."
Wainwright's BBC One period drama Gentleman Jack, which stars Suranne Jones, was dropped by production partner HBO in June, although the BBC is keen to continue making it if a new partner can be found.
The screenwriter appeared to refer to it as she said: "I just had six months off because I had quite a bruising year and I'd worked non-stop for the last 20 years, but I quite enjoyed the time off and I'm going back to work in February."
She said her new project was still top secret, but teased that "hopefully it'll be quite exciting".
Happy Valley's final episode airs on Sunday at 9pm on BBC One.