Mellor, who is responsible for such audience-grabbing TV dramas as Band Of Gold, Playing The Field and Fat Friends, was originally an actress before she concentrated on writing.
In her latest drama she plays Rita - a working class mum who witnesses a murder, then is forced to go on the run with her daughter and elderly neighbour when the police fail to protect her from gangsters.
But they have no money so Rita comes up with an elaborate shoplifting scam, involving a prestigious department store, to enable the trio to survive.
Mellor, 48, says: "I wrote A Good Thief quite some time ago. I had written it with Julie Walters in mind but she couldn’t do it because of other work.
"Then a couple of years ago I appeared in a one woman show at the West Yorkshire Playhouse - it was terrifying. Unknown to me Pippa Cross from Granada TV, who originally commissioned A Good Thief, came to see the show.
"Two months later Pippa suggested I might like to revisit the project and get involved in another way as well as writing. I knew I didn’t want to direct so on the spur of the moment I suggested I play Rita. She told me that she had hoped I would say that."
Although she sailed through the screen test and was encouraged to tread the boards once more by her actress daughter Gaynor Faye, who is still remembered as Coronation Street’s Judy Mallet, Mellor says it wasn’t an easy decision.
"At first I had a terribly guilty conscience. I thought, ‘What about all those actresses I am doing out of work?’. But then I thought about women like Victoria Wood and Caroline Aherne who appear in their own productions.
"I also thought that, over the years, I must have written thousands of roles which had given actresses work. Gaynor told me that she thought I had earned the part so go ahead.
"It was ten years since I was last on screen and when I saw the finished drama the first thing I thought was that I looked like my mum.
"But people have been very kind and complimented me since - I think they were genuine. Now it is time for it to be shown on TV I am getting nervous. I am feeling a little vulnerable as I am the writer and lead actress."
It seems Mellor has developed a taste for acting once more as she has just finished filming another TV drama called Stan The Man, where she plays the mother of her real life daughter Gaynor.
Mellor explains: "Gaynor went up for the part. She telephoned to tell me she had got it and said, ‘Guess what? They want you to play my mum’.
"But it’s not such a big part as Rita and I didn’t write it so it was fantastic fun. It has done wonders for my confidence and made me believe I can act again.
"I also got to travel over to Manchester every day with Gaynor, which meant we could see much more of each other. It was fantastic having her on set as I knew she was rooting for me."
But her return to acting does not mean that Mellor is turning her back on her writing talents. She has just finished a second series of Fat Friends which will be screened in the autumn.
Mellor says her concern about the way society looked at people’s shapes had originally compelled her to come up with the drama based around a slimming club.
She says: "I think as a society we are obsessed with our shape. But it is a basic human trait that we are all going to be different shapes and sizes - that’s the way it should be.
"People should be allowed to live their lives how they want and not be preached at about their size. It is terrible when you hear seven-year-old girls saying they are not happy with their bodies - I wanted to address those issues.
"You can guarantee that within half an hour of talking to somebody they will have told you that they are on a diet or unhappy about their weight. We are totally preoccupied with our weight."
Bought up on a Leeds council estate, Mellor still lives in the Yorkshire city. She married Anthony when they were young and by the time she was 19 had two children.
She studied drama as a mature student at Wakefield’s Bretton Hall then landed a part in the short-lived northern soap Albion Market. Meantime, her husband Anthony had returned to college himself - he was originally a garage mechanic but he now runs a daycare centre.
One day Mellor came up with an idea for an Albion Market story. She left the script on a desk at Granada TV. Somebody read it and liked it. She started writing ‘bits’ for Coronation Street and Brookside before Band Of Gold.
Ironically, she could have stayed an actress as she landed a part in well-known soap Emmerdale, but didn’t take it.
She says: "I can’t remember what the part was, but it was for a long-running character. I remember calling my acting agent to tell her I was going to concentrate on writing - I don’t think she was very happy.
"If I had made a complete mess of things, it would be different, but I think I made the right choice."
l A Good Thief is on Scottish on Monday at 9pm