Despite Stephen was a foreigner, who held no Venetian citizenship, the Great Council of Venice approved his marriage with Tomasina, probably because in this way they could have offset the Hungarian claims over Dalmatia.

Tomasina was a huge admirer of celebrations and singing performances, according to Casimir Freschot, who also styled her "Queen of Adriatic", a "new Esther" and "Violet Lady" in his chronicle.

He was elected and crowned king of Hungary shortly thereafter, but it took him more than a year to consolidate his power against Albert I, Duke of Austria and the rival claimant Capetian House of Anjou.

Meanwhile, the Republic of Venice decided to send an official delegation to Hungary, headed by her maternal uncle, Giovanni Cornaro, to accompany Tomasina to the kingdom in September 1291.

However, the ships of the delegation spent a long time wandering off the coast of Dalmatia in the following months, because of the piracy activity of the Šubići, partisans of the rival Angevins.

In May 1292, the Great Council instructed the delegation, that if they cannot continue their journey within weeks (it is plausible that they were unable to leave Zadar securely), they should return to the lagoons of Venice.

[9][10] The delegation was finally able to continue their journey and Andrew III sent his confidant Ugrin Csák to Primorje in order to escort his mother Tomasina to Hungary, but he was captured and imprisoned by certain "disloyal barons".

[9] Initially, Tomasina had no official political role in Andrew's court and she was frequently styled as simply "the king's mother" (Latin: mater domini regis) in 1293.

[16] Only a non-authentic charter refers to her as "Duchess of Whole Slavonia and Lady of the Maritime Province" (Latin: ducissa totius Sclavonie et maritimarum partium principissa) with the date 1293.

[12] In this document, Andrew III made a gesture by naming Paul Šubić the hereditary ban of Croatia and Dalmatia on condition of faithful service to the king and his mother.

[10] She styled himself simply "the king's mother" still in early 1295, when donated the estate Szob – a portion of the Segesd lordship prior to that – to ispán Ladislaus Tengerdi.

[17] Tomasina's full title was the "Duchess of the Whole of Slavonia and the Governor of the Provinces from the Danube to the Adriatic Coast" (Latin: ducissa totius Sclavonie et gubernatrix partium citra danubialium usque maritima); her duchess province covered the region "Slavonia" between the rivers Danube and Drava, through from Gvozd Mountain to the Adriatic Sea, while she administered the remaining attached territories with the title of governess.

[20] Tomasina ruled her province with sovereign authority and was mandated to defeat those powerful local lords who had aspirated to establish oligarchic domains in the region.

Eyza plundered the region of Baranya County, his men, John and Paul Véki, looted and destroyed Tomasina's personal estate Danóc (present-day a borough of Gajić in Croatia)[25] too with its nearby monastery in Gecs, both defended by Provost Andronicus of Veszprém.

In response, Tomasina Morosini recruited an army; Palatine Nicholas Kőszegi, Paul Balog, Bishop of Pécs and Ladislaus Tengerdi also participated in the military campaign.

Radoslav Babonić also rebelled against the duchess in the same year; as a result she besieged and captured his castle of Orbászkő (ruins near present-day Vrbaška, Bosnia and Herzegovina).

[23][26][27][28] Tomasina also sent a reinforcement unit to the royal army of Andrew III, who marched to Tiszántúl to defeat Roland Borsa, the powerful Voivode of Transylvania.