Tomasevich was born in the Kingdom of Dalmatia, then part of Austria-Hungary, and after completing high school and attending a commercial academy, he earned a doctorate in economics from the University of Basel in Switzerland.

During World War II, Tomasevich worked for the Board of Economic Warfare and the United Nations Relief and Rehabilitation Administration, and post-war he joined the Federal Reserve Bank in San Francisco.

Between 1943 and 1955, Tomasevich published two positively reviewed books on economic matters; one focused on marine resources and the other on the peasant economy of Yugoslavia.

His final book was the second volume of the series – War and Revolution in Yugoslavia 1941–1945: Occupation and Collaboration – which was edited by his daughter Neda Tomasevich then published posthumously in 2001.

In his obituary by Alexander Vucinich in the Slavic Review, Tomasevich was described as "a master of scholarly skills, a person of bountiful erudition, wit and human dignity".

[9] Tomašević later described his family as have been in a fairly comfortable financial situation, which enabled them to send him to high school in Mostar, after which he studied for four years at a commercial academy in Sarajevo.

[7] During several years working in Belgrade[10] – where he was employed as a financial expert at the National Bank of Yugoslavia[11] – and Zagreb, Tomašević saved enough money to travel to Switzerland to study at the University of Basel where he earned a doctorate in economics in 1937.

[16] According to his obituary in the Slavic Review written by the historian Alexander Vucinich, Tomasevich "gave his lectures rich and pertinent content, precise organization and warm delivery".

In 1976, Tomasevich contributed an essay to a book in which he presented a striking historical and sociological examination of his extended family reaching back to the early nineteenth century.

He describes Tomasevich as having "a temperament that encourages inner discipline ... he gave undivided attention to each of the research projects until full completion had been achieved".

[20] According to Vucinich, the book "provided a solid and much-cited analysis of government efforts to stabilize national finances at the outset of the Great Depression".

[21] His 1938 treatise Novac i kredit (Money and Credit), "helped train an entire generation of Yugoslav financial experts", according to Vucinich.

[24] Vucinich described this work as "a highly competent inquiry into international relations in the Pacific basin centered on an issue of vital economic importance".

[1] Vucinich observed that the book was an "impressive testimony to Tomasevich's ability both to penetrate the depths of messages carried by documentary material and to be scrupulously careful in drawing conclusions".

[1] He concluded that Tomasevich had been "eminently successful in placing the economic problems of the Yugoslav peasantry within a larger social, political and historical framework".

[31] Auty described the work as a "most impressive ... scholarly examination ... of evidence",[30] that was meticulously referenced, and a "deceptively lucid account of a most complex and difficult subject".

While he identified a few shortcomings and minor errors of fact, Goldstein described Tomasevich's work as a complete and lucid description and explanation of Yugoslavia's occupying and collaborationist forces.

"[32] In a review of the book published the following year, the Royal Military Academy Sandhurst lecturer and German historian Klaus Schmider described Tomasevich's grasp of the sources in five languages as "stupendous",[38] and he observed that the result was well worth the twenty-six-year wait between the volumes.

[38] According to Vucinich, Tomasevich was "a master of scholarly skills, a person of bountiful erudition, wit and human dignity, and a leading expert on the economic and social history of the former Yugoslavia".