On 13 December 1769, Leopold and Wolfgang set out from Salzburg, armed with testimonials and letters that Leopold hoped would smooth their passage. Among the most important was an introduction to Count Karl Joseph Firmian of Milan, described as the "King of Milan", an influential and cultivated patron of the arts. His support would be vital to the success of the entire Italian undertaking.
The pair travelled through Innsbruck, then due south to the Brenner Pass into Italy. They continued through Bolzano and Rovereto to Verona and Mantua, before turning west towards Milan. Leopold's financial plans for the journey were broadly the same as for the family's grand tour—travel and accommodation costs were to be met by concert proceeds. This 350-mile winter journey to Milan occupied a difficult and unpleasant six weeks, with the weather forcing extended stops. Leopold complained in his letters home about unheated inn rooms: "... freezing like a dog, everything I touch is ice". Early concert receipts were modest; according to Leopold, costs were running at around 50 florins a week. After some unwise boasting about profits during the grand tour, Leopold was now more cautious about revealing financial details. He tended to emphasise his expenses and minimise his takings, writing, for example: "... On the whole we shall not make much in Italy ... one must generally accept admiration and bravos as payment."
The pair arrived in Milan on 23 January and found comfortable lodgings in the monastery of San Marco, not far from Count Firmian's palace. While they waited to see the Count, they attended Nicola Piccini's opera Cesare in Egitto. Firmian eventually welcomed them with generous hospitality and friendship, presenting Wolfgang with a complete edition of the works of Pietro Metastasio, Italy's leading dramatic writer and librettist. Firmian also hosted a series of concerts attended by many of the city's notables, including the Habsburg Archduke Ferdinand, a possible future patron for the young composer. For the last of these occasions, Wolfgang wrote a set of arias using Metastasio's texts. These were so well received that Firmian commissioned Wolfgang to write the opening opera for the following winter's carnival season in Milan, just as Leopold had hoped he might. Wolfgang would receive a fee of around 500 florins, and free lodgings during the writing and rehearsal. The Mozarts left Milan on 15 March, heading south towards Florence and Rome, committed to return in the autumn and taking with them fresh letters of recommendation from Firmian.