At the age of fifteen, Stojanović became an activist in a group of student organizations called Young Bosnia, which strongly opposed Austria-Hungary's occupation of Bosnia-Herzegovina.

After the war, his service to the Partisan cause was commemorated by the construction of a memorial in Prijedor, the naming of streets, public buildings and a park after him, in song and in film.

Mladen Stojanović's maternal grandfather was a Serbian Orthodox priest from Dubica, Teodor Vujasinović;[2] he had participated in Pecija's revolt against the Ottoman Empire.

Although the provincial government imposed the name "Bosnian" on the language of the province (Serbo-Croatian), the students demonstratively termed it as Serbian or Croatian depending on their ethnic affiliation.

[7] In early-to-mid 1912, Stojanović and his schoolmate Todor Ilić joined Narodna Odbrana (National Defence),[5] an association founded in Serbia in December 1908 on the initiative of Branislav Nušić.

They stayed for several days in Belgrade, the capital of Serbia, where they met Gavrilo Princip, another activist of Young Bosnia who was also a member of National Defence.

Stojanović and Ilić then spent a month at army barracks in Vranje in southern Serbia, undergoing military training under the command of Vojin Popović, a famous Chetnik guerrilla fighter.

They held a series of lectures for the members of the association, explaining their views on the current political situation, and promoting the unity of South Slavic peoples in their struggle to liberate themselves from Austria-Hungary.

[21] Shortly after the assassination, Stojanović wrote in his notebook a quote from Giuseppe Mazzini: "There is no more sacred thing in the world than the duty of a conspirator, who becomes an avenger of humanity and the apostle of permanent natural laws.

The Austrians became aware of this because their army temporarily took Loznica in western Serbia at the beginning of World War I, and there they found National Defence documents containing records of all Bosnians that attended the training.

In villages around Prijedor, where brawls were common, rowdies sang about him:[26] Udri baja nek palija ječi, ima Mladen što delije liječi.

[37] Resistance began to emerge in occupied Yugoslavia; royalists and Serbian nationalists under the leadership of then-Colonel Draža Mihailović founded the Ravna Gora Movement, whose members were called Chetniks.

In response to this appeal, the leaders of the KPJ decided on 4 July in Belgrade to launch a nationwide armed uprising,[41] which began three days later in western Serbia.

[54] On the morning of 19 July 1941, Stojanović and Bašić arrived at the camp of the communists and their sympathisers who escaped from Prijedor, situated at Rajlića Kosa above the village of Malo Palančište.

[52] On the night of 25 July 1941, at Orlovci, Stojanović and seven other leading communists of Kozara had a meeting with Đuro Pucar, the head of the KPJ Regional Committee for Bosanska Krajina.

[63] The sawmill and its stored products—including a large quantity of railroad ties, with which the Germans were allegedly planning to repair railways destroyed by Soviet partisans in occupied Ukraine—were burnt down.

[66] In the villages of Kozara, people sang about Stojanović:[69] Ide Mladen vodi partizane Razveo ih na sve čet'ri strane ...

[74] According to the writer Branko Ćopić, who was a Partisan in Grmeč, Stojanović was greeted by a crowd of villagers and welcomed with the traditional bread and salt ceremony when he crossed the Sana River.

[79] The Partisan leadership of Bosnia-Herzegovina considered that Stojanović had successfully countered the Italian propaganda and improved the condition of the 1st Krajina Detachment during his tour.

[72] Stojanović left Grmeč and went to Skender Vakuf in northwest central Bosnia to participate in the first regional conference of the KPJ in Bosanska Krajina,[75] which was held from 21 to 23 February 1942.

[63][72] At the conference, Stojanović was appointed to lead a unified command of Partisan forces in Bosanska Krajina,[80] but on 24 February he was replaced with Kosta Nađ.

On 5 March, Stojanović, Nađ, and Danko Mitrov (the commander of the 4th Krajina Detachment) set out for Lipovac with the Kozara Proletarian Company,[85] an assault unit formed in February 1942.

The two headquarters and the field hospital were attacked on the night of 31 March by members of the Jošavka Partisan Company, who had joined the Chetnik side under the influence and leadership of Radoslav "Rade" Radić, the deputy commissar of the 4th Krajina Detachment.

Through a messenger, Radić told Stojanović to write a letter ordering Danko Mitrov to remove all Partisan units from the area around Jošavka.

At the 2012 commemoration, the president of the Partisan War Veterans' Association of Republika Srpska declared:[98] Mladen je bio čovjek za primjer, revolucionar od najranije mladosti pa do kraja života, najpopularnija ličnost ustanka na Kozari, Krajini i mnogo šire i jedan od najhrabrijih boraca i rukovodilaca Narodnooslobodilačke borbe.

Mladen was a model person, a revolutionary from his early youth to the end of his life, the most popular figure of the uprising in Kozara, Krajina, and a much wider area, and one of the bravest fighters and leaders of the National Liberation War.

[98]In his youth, Stojanović wrote poems, only one of which is published—in a 1918 issue of the literary magazine Književni jug,[11][99] whose editor was future Nobel Prize winner Ivo Andrić.

According to the poet Dragan Kolundžija, Stojanović's poems are lyrical miniatures composed in free verse, focused on man and nature, and filled with melancholy.

[100] Stojanović's poetic inclinations were manifested in his letters to his wife Mira Stojanović, especially when he writes about his patients:[101] I, kad se podižu i osjećaju strujanje snage i proljeća u svojim žilama ja kao da dolazim sebi, ostavlja me neki zanos i ja tražim druge bolesne oči djece, žena, majki, staraca; nalazim ih i ponovo zaboravljam sve.

And, as they rise and feel the stream of power and spring in their veins, I seem to come to [as though] some kind of ecstasy leaves me, and I look for the other ailing eyes of children, women, mothers, old men; I find them and again I become oblivious to everything [else].