[3] Mazzo is in the upper part of Valtellina in an area of primary importance: laying at the foot of the Mortirolo Pass, it allowed easy access to the Valcamonica valley and subsequently to the territories of the Republic of Venice, which had strategic interests in trade with the German-speaking countries.

With the disintegration of the Western Roman Empire and the Barbarian invasions, Mazzo was probably incorporated, after 489, into the Ostrogothic Kingdom of Theodoric the Great.

To name just a few commonly used words, we can mention "sberlüsc" (lightning), "güdàzz" (godfather), "sluzz / slòz" (wet), "balòss" (smart, rascal), "maschérpa" (ricotta), "gnècch" (in a bad mood), "bütér" (butter), "scràna" (bench), "stracch" (tired), "trincà" (to drink), "sgrafignà" (to steal), "grignà" (to laugh), "biótt" (naked), "rüt" (rubbish, garbage), "bródeg "(dirty), "ghèi" (money).

[4] The first written records of Mazzo date back to 795, where the presence of the homonymous parish (pieve), which extended from Sernio to Sondalo, was mentioned by the emperor of the Franks Lothair.

[4] After the year 1000, the parish of Mazzo dedicated to Saint Stephen played a fundamental role in the spread of the Christian faith in Valtellina (together with others including that of Samolaco, Chiavenna, Ardenno, Teglio, Bormio and Sondrio).

In the 11th century, a branch of the noble family of de Mazia or von Matsch from Val Venosta moved to Valtellina,[7] entrusted by the emperor Henry IV with the task of keeping the Alpine passes open for the passage of imperial troops.

During the mid-15th century, they were replaced by the Sforza, who had to face the first attempts of the Swiss Three Gray Leagues to dominate the valley.

After the fall of Ludovico il Moro and a short French rule, the entire valley was conquered in 1513 by the Swiss leagues and remained under their jurisdiction until 1797, when it was annexed to the newborn Cisalpine Republic together with a large part of Lombardy and Emilia-Romagna.

A recent renovation revealed that the outside walls were covered with ancient frescoes depicting coats of arms, inscriptions, zodiac signs and the representation of the Months.

The carved main door is from 1597; the frame around it was made by Bernardino da Maroggia with white marble in 1508.

The white marble altar replaced a precious triptych made with gilded wood currently on display at the Poldi Pezzoli Museum in Milan.

On the external façade, we can see a fresco from Giambattista Muttoni, while inside paintings from Francesco Piatti represent four episodes of the Old and New Testament.

[8] The 15th-century painting on a house in Vicolo della Torre, which depicts scenes of battles with camels and elephants, tropical plants and oriental figures, attests to the connection of Mazzo with the Republic of Venice, whose merchants crossed the Mortirolo Pass to reach the Raetia.

The large hall on the ground floor, the "Salone degli Stemmi", was frescoed in 1543 with a heraldic frieze representing the coats of arms of families linked, by kinship or friendship, to the owners.

[8] This area was in fact of strategic importance as it allowed to control the two accesses to Bormio (namely Valgrosina and Valdisotto) and the movements of merchants and armies to and from the German-speaking countries.

Of it, only the four-storey rectangular tower has remained, with arrow-loops and windows through which archers would launch arrows to defend the castle.

[8][10][4] The economy until the sixties was mainly agricultural (cereals, potatoes, vegetables, fodder and fruit) and based on cattle breeding.