McCarthy will work primarily with Manchester United's strikers as Erik Ten Hag prepares for his first season in the Old Trafford dugout.
The 44-year-old was most recently in charge of South African side Amazulu, leading them into the group stages of the CAF Champions League for the first time in their history.
He left Amazulu in March this year, having also managed Cape Town City between 2017 and 2019.
McCarthy had an illustrious playing career, representing the likes of Ajax Amsterdam, Celta Vigo, Blackburn Rovers and West Ham United but he is best remembered for his spell at FC Porto, where he won two Portuguese league titles and the 2004 UEFA Champions League under Jose Mourinho.
A statement on the Manchester United website read: "Manchester United boss Erik ten Hag has added to his backroom team with the appointment of Benni McCarthy as a first-team coach."
"The former South Africa striker was a boyhood Reds fan and scored twice against us for Porto in the 2003/04 Champions League knockout stages.
"The ex-forward will specialise in coaching attacking plays and positioning.
"Everybody would like to extend a warm welcome to Benni, upon joining the club."
McCarthy remains South Africa's all-time leading goalcorer with 31 goals in 79 appearances.
