Mr Nosey is one of the popular characters
More than 100m books have been sold since 1971, making Roger Hargreaves the second best-selling author to Harry Potter creator JK Rowling.
The licensing agreements on the series alone are worth an estimated £130m to the Kent-based business.
To mark the anniversary a new character has been born, Mr Cheeky.
The Mr Men empire began when Mr Hargreaves' son Adam, then aged seven, asked what a tickle looked like.
From there, the little round figure, with impossibly long arms and an urge to tickle anyone within reach, was born.
The irrepressible Mr Tickle paved the way for 42 more funny men, including Messrs Happy, Nosey, Greedy and Snow.
The success of the series enabled Mr Hargreaves to turn his back on the advertising world and work from home, in Kent.
Adam, now 38, said the simplicity of the characters was the key to their success.
He said: "My father was very successful in capturing basic human characteristics and turning them into something humorous.
The Mr Men were later followed by the Little Misses, with Miss Bossy taking the lead.
Adam continued writing books, drawing on an inherited sense of humour and creativity, after his father died unexpectedly of a stroke 13 years ago, aged 53.
His sister, Amelia Beddoe, later joined the business, helping run the Mr Men empire based in Tunbridge Wells.
The rights to the characters however, still belong to Mr Hargreaves' widow Chris.
And for those not content to own just the books, Mr Bump plasters, Mr Perfect boxer shorts and Little Miss Naughty underwear, are among the vast range of merchandise.
Mr Cheeky was created by eight-year-old Gemma Almond, from Shropshire, who won a nationwide competition.