After entering the service of the Polish King at a young age and leading a successful military career, he became a rittmaster of the obrona potoczna, and later the starosta of Bar and Ulaniv [uk] (1520–1552) and Terebovlia (1552–1561).

Also in 1539 Queen Bona provided for further benefits: It was left to Pretwicz city and Scharawka Castle (55 km northwest of Bar) with all its goods for lifelong use, from 1550 as property.

As a starost in the confines of Poland, Bernard Pretwicz's main mission was to protect Podolia from Tatar expeditions that devastated the region every year.

The resources of the starosty being insufficient, it engaged many Cossacks who often paid themselves by plundering the lands of the sultan, sometimes supplying them weapons and clothes.

Both in peace and in wartime, the Starost was the district captain, so in addition to his post as head of administration at the same time military Commander in Chief.

Pretwicz became "the man of providence for the Podolian lands" (source: Pulaski ): The small fortress bar offered shelter and food for only 30 men.

From Cheremissen and Cossacks he formed his own troop of about 300 men, which he housed in smaller, well-mounted groups in castle-like fortifications in villages in the area.

Pretwicz became "the man of providence for the Podolian lands" (source: Pulaski ): The small fortress bar offered shelter and food for only 30 men.

From Cheremissen and Cossacks he formed his own troop of about 300 men, which he housed in smaller, well-mounted groups in castle-like fortifications in villages in the area.

Pretwicz confronted them with a small group of Cossacks, drove them about 100 km to Ochakiv in the Jedisan, at that time one of the most important permanent places of the Ottoman Empire on the Black Sea, took away their rich prey and returned with Tatar women and children as prisoners.

In 1550, the Tartars, this time together with the Wallachians, re-entered the Polish Podolia and besieged the fortress bar, well equipped with weapons, with at least 56 large and 1,120 small barracks.

Later, when King Sigismund II August, out of political calculations, prohibited him from retaliating against the Tartars, Pretwicz instead provided better border and city fortifications within his area of responsibility.

Thus, under the protection of the "glorious starost" Pretwicz, the entire region of Bar and Vinnytsia was populated, trade and agriculture began to flourish.

Pretwicz had probably met Duke Albrecht of Prussia, whose mother Sophie was the sister of King Sigismund I, perhaps at the Polish court.

For example, a grandson of Biecki, who served as King Sigismund I as a squire, explicitly asked for permission to learn equestrian service at Pretwicz.

For his military achievements also speaks that only two years after Pretwicz 'death King Sigismund II in Poland a standing army erected to meet the renewed destruction in the border provinces by the Tartars, and long after his death the rhyme "Za Pana Pretfica wolna od Tatar granica" (At Pretwicz times the border was free of Tatars) was very popular in Poland.