The brothers were born in Karlovac, Croatia (at the time part of Austria-Hungary). After receiving a basic education there, they started traveling across Europe. By 1898, the older Mirko walked the distance between Paris in France and Saint Petersburg in Russia. It is believed that the brothers started the journey after being impressed by the books or lectures of another Croatian explorer, Dragutin Lerman, who was in Africa as one of the leaders of Henry Morton Stanley's team. The theory that the influence, support, and encouragement of the Russian government, which was interested in Africa at the time, also had an effect cannot be denied.
They started in January 1899, leaving Karlovac for Trieste. They visited Alexandria and Cairo, then walked along the Nile to Sudan. After returning to Cairo, they headed to Port Said, Djibouti, and Aden, reaching Bab el Mandeb. After traveling to Obock, they returned to Djibouti, but eventually ended up in Harar and then Addis Ababa in Ethiopia.
Emperor Menelik II of Ethiopia provided an escort of 300 people to continue their expedition to the south. Mirko became the governor of provinces with his older brother, Stjepan, as his lieutenant. For three years they set the lines between Ethiopia and Kenya, bringing the tribes under unified and central government in Adis Ababa. Near Lake Turkana (Lake Rudolf), their expedition and military camps were named Seljenville.
The greatest benefit of the journey was that they expanded the sovereignty of Ethiopia to the right side of the river Omo. The current borders between the states of Ethiopia, Sudan, and Kenya around lake Turkana are the same as those drafted by the brothers. In 1902, due to political tension, they left, only to return after the end of hostilities between imperial Britain and France (cf. Fashoda Incident).
In April of 1903 the two brothers left for Rio de Janeiro in Brazil with the intention of entering areas that were at that time unknown, to establish traveling and trade routes and permanent posts. The government of Brazil was interested in enhancing trading and development going from the east to the west. During 1903, they established the Croatian Scientific Mission (Mision Cientifica Croata). They made contacts with the Guarani Indian tribe around the Aguapehu river and intensively mapped the area. At the end of 1903 they were on their way from Salta Grande to Asunción in Paraguay, ending their journey on the Paraná River in Argentina. The expedition is described in the book El Salto del Guayra, published in 1905 in Buenos Aires. In 1905 they left for the waterfalls of Iguaçu and later conducted research for the state of Santa Catarina.
With the beginning of 1905, after crossing the Andes by foot and with mules from Argentina, the Seljans arrived in Chile. In Valparaíso as well in Santiago they were planning a new expedition approaching the area of the Amazon River from the south side. Here, they met immigrants from the old country and established the Sociedad Slava Explotadora de Mato Grosso (Society for the Exploration of Mato Grosso), which was supposed to provide public relations and support expeditions. In May of the same year, they left for Montevideo in Uruguay with their new friend and new explorer Franjo Pommer. Since more time was needed to be properly ready for the journey, they actually started rather late, in 1908 focusing on the Rio Branco and Rio Maraca rivers in the Amazon delta. Exhausted and discouraged by delays in obtaining permits from the government, they returned to Chile hoping to see Peru and Bolivia. They passed 1911 exploring the Atacama desert and the area of Tarapaca. In that year, for the first time, they entered Peru. They established Hrvatsko dionicarsko drustvo (Croatian Brokerage Society) with the purpose of helping the government of Peru to plan, finance and build roads connecting two parts of the country, from the Andes to the Amazon basin.
The brothers for the first time took different ways in October 1912 with Stjepan heading toward the United States to initiate new business contracts, while Mirko was supposed to continue their research, starting the new expedition. Because his group did not meet another group of explorers to continue with, Mirko decided to proceed deeper into the rainforest around Huayabamba river, on his own, not to be seen again. In the aftermath, numerous theories, stating murder, loss, even cannibalism, came around, but none was ever proven. After his older brother's disappearance, Stjepan kept moving, searching for mineral deposits in south and central America until, in 1917, he decided to settle and made his home in Ouro Preto, Brazil, where he died in 1936.
It is obvious today that the journeys of the two explorers were partly influenced or disrupted by world politics and by newly industrialized society. During their travels, they conducted numerous observations and provided documentation later used for works in geomorphology, climatology, and ethnography. Their initial intention was to enrich their lives and the world with new knowledge and understanding. In their diaries, they described the areas they passed through, discussed Indian tribes they met, and left numerous sketches, photographs, maps, data, and memories regarding the nature and cultures that they had so much enjoyed meeting.