Mourinho left Real Madrid in 2013 to manage English club Chelsea for a second spell before signing with United in 2016.
"A couple of years later I was informed that an investigation had been opened".
According to EFE and ESPN FC sources, the case is not yet closed and further court dates are pending as the country's tax agency, Hacienda, decides whether there is enough new evidence to determine that the previous settlement was not enough and whether Mourinho must pay more.
Meanwhile, the case has come just days before United go up against Chelsea in the Premier League.
"That's why I've been here five minutes today to say to the judge exactly what I'm telling you".
Mourinho is the latest hihg-profile football figure to face the court over unpaid taxes in Spain recently.
Red Devils coach was seen entering to the courthouse of Pozuelo de Alarcon.
Other clients at Mendes' Gestifute agency including Fabio Coentrao, Pepe, Ricardo Carvalho and Angel Di Maria have also had image rights issues with the Spanish tax authorities.
The Portuguese manager owed Spanish tax authorities 3.3 million euros (2.92 million pounds).
Mourinho arrived at the local court at around 9.30 am in Madrid for a 10 am hearing.
"The Spanish Government in turn, through the Tax Department, issued a certificate in which it attested that he had regularised his position and was in compliance with all his tax obligations".
And this is important for United fans, who will know how the mood of their boss can so easily permeate the entire first team squad. Last year, Lionel Messi and his father were found guilty on three counts of defrauding tax authorities of 4.1 million euros ($4.7 million) from income made from image rights.