Nevertheless, after his father's death in 1312 he received the town of Siewierz, but he continued his church career and in 1313 he joined to the Knights Hospitaller.

Two years later (in 1315) Mieszko, together with his older brother Bolesław, traveled to Hungary, at the request of their sister Queen Maria and soon after his arrival, Mieszko was appointed Hungarian Prior of the Knights Hospitaller; however, three years later, in 1318, he resigned from that dignity at the express command of the Pope.

This happened despite the opposition of the local Chapter, who were completely against the nepotism; however, the full support shown to the Piast princes by the King Charles I Robert and his new wife Queen Elizabeth (Queen Maria died in 1317) ended this dispute with a complete success to Mieszko, who, in order to assume the rule over the bishopric, finally decided to resign his Duchy of Siewierz on behalf of his brother Władysław.

In 1334, and after years of continuous conflict with the Chapter, Mieszko was named Bishop of Veszprém (this time with the consent of the Pope and the local nobility).

During this time, he also held the position of chancellor of Queen Elisabeth of Hungary (who certainly was involved in the election of Mieszko as Bishop), who bought him important benefits (Mieszko received from Federal Chancellery the amount of 500 fines per year by orders of Queen Elisabeth).