The 43-year-old Wigley, originally from Ashton, East Manchester, has become a highly-regarded coach in his time on the south coast, moving up the youth ranks to take charge of the reserves and then the first team.
But Wigley did not have the best of times in charge at St Mary’s and stepped down in December after a disappointing four months.
He then returned to Southampton in March, leading their U18 Academy side to the southern area title by 23 points, convincing new City boss Stuart Pearce that his is the man to help City’s productive Academy continue its rapid development.
“Steve is an excellent addition to my team,” he told the City website. “Not only does he have experience as a Premiership manager, but he has experience at all levels of football.
“He is enthusiastic, a real team player, and everything I’m looking for in a coach. He'll be a great link between the Academy and the first team.”
“City is most definitely a club on the up,” added Wigley. “As a local boy, it’s made my decision even easier, and I can’t wait to join Stuart and his team, and experience all that Manchester City has to offer.”
A speedy winger in his playing days, Wigley played alongside Pearce at Forest and also had spells with Sheffield United, Birmingham, Portsmouth and Exeter.
He went straight into coaching after retiring, and has also been assistant Academy director at Forest before joining Southampton in 2001.
Hartford, 54, has left after 10 years on the Blues’ coaching staff. The Scot, who made more than 320 appearances for City in two spells in the 70s and 80s, returned to the club as assistant manager to Alan Ball in 1995.
He stepped in as caretaker boss when Ball left the following year, and then carried on as coach until taking charge of the reserves in more recent years.
“Asa has been with us for many years and has done a good job,” said chairman John Wardle. “He will always be very welcome at City, he has been a great servant but the manager felt he wanted to bring in a new face.
“The club has got to move onwards and forwards, and that sometimes means a change in personnel.”