After a restless period of travelling Petőfi attended college at Pápa, where he met Mór Jókai, and a year later, in 1842, his poem A borozó ("The Wine Drinker") was first published in Athenaeum under the name Sándor Petrovics. On 3 November the same year he published this poem, using the name "Petőfi" for the first time. However, Petőfi was more interested in the theater. In 1842 he joined a traveling theater, but then had to leave it. He tried to keep himself financially afloat by writing for a newspaper, but that wasn't enough. Malnourished and sick, he arrived in Debrecen, where his friends helped him get back on his feet.

In 1844 he walked from Debrecen to Pest to find a publisher for his poems, in which he succeeded, and the poems were becoming increasingly popular. He relied on folkloric elements and popular, traditional song-like verses. Among his longer works is the epic János Vitéz ("Sir John", 1845). On the other hand, he felt he was forced into a folkish, wine-and-pubs, low-quality niche by his publisher, while in fact he also had an extensive Western-oriented education and revolutionary passions to write about. (Of course, these would be difficult to publish, due to the heavy censorship of the time).

In 1846, he met Júlia Szendrey in Transylvania, and they married the next year, against the will of her father, spending their honeymoon in the castle of Count Sándor Teleki, the only aristocrat among Petőfi's friends. Afterwards, he was even more possessed by thoughts of a global revolution. He moved to Pest and joined a group of like-minded students and intellectuals who regularly met at Café Pilvax. They worked at promoting Hungarian as a language of literature and theatre. (The first permanent theatre (the National Theatre) performing in Hungarian opened at this time.)

15 March 1848 was Petőfi's day. Among the various leaders of the revolution - called Márciusi Ifjak ("Youths of March") - Petőfi was the key in starting the revolution in Pest, co-author and, respectively, author of the two most important written documents: the 12 Pont (demands to the Habsburg Governor-General) and the Nemzeti Dal. When the news of the revolution in Vienna reached them on the 15th, Petőfi and his friends decided to change the date of the "National Assembly" (a rally where a petition to the Hungarian noblemen's assembly would be approved by the people), from 19 March to the 15th. (This was a lucky decision, given that the authorities knew their plans, and intended to arrest the revolutionaries on the 18th.) On the morning of the 15th, the revolutionaries with Petőfi began to march around the city of Pest, reading the poem and the 12 points to the crowd (which swelled to thousands). Then, they visited printing presses, declaring an end to all forms of censorship and printing Petőfi's poem together with the 12 Pont. The mayor was forced by the crowds to sign the 12 Pont. Later, a mass demonstration was held in front of the newly-built National Museum, after which the group left for Buda on the other bank of the Danube. When the crowd rallied in front of the Imperial governing council, the representatives of Emperor Ferdinand felt they have no choice but to sign the 12 points. As one of the points was freedom for political prisoners, the crowd moved on to greet the newly freed revolutionary poet Mihály Táncsics.

Petőfi's popularity waned as the memory of the glorious day faded, and the revolution went the way of high politics: to the leadership of the nobles. Those in the noblemen's Assembly in Pozsony, (today Bratislava) had in fact been pushing for slower reforms at the same time - delivering a list of demands to the Emperor on the 13th - but events had overtaken them briefly. Petőfi disagreed with the Assembly, and criticised the way they saw the goals and methods of the Revolution. (His colleague Táncsics was imprisoned yet again by the new government.) In the general election, he ran in his native area, but did not get the seat. At this time, he wrote his most serious poem, Az Apostol ("The Apostle"), an epic about a fictional revolutionary who, after much suffering, attempts, but fails, to assassinate a fictitious king.

Petőfi joined Polish revolutionary general Józef Bem's Transylvanian army, fighting a successful campaign against Habsburg troops, Hungarian, Romanian and Transylvanian Saxon militias. However, it was defeated repeatedly when Imperial Russia intervened to aid the Austrians. He was last seen in the battle of Segesvár (Sighişoara) on 31 July 1849, where it is generally accepted he was killed while fighting opposite Russian forces. A Russian military doctor recorded an account of Petőfi's death in his diary. He saw an unusual looking corpse killed by lance wound to the stomach; the body had Petőfi's characteristic yellowish face and matching clothing; Petőfi had the habit of wearing a civilian jacket with uniform trousers. As his body was never officially found, rumours of his survival persisted: to some who refused to believe he had died, he was one of several pseudo-Petőfis who roamed the land for many years; others continue to believe he was taken prisoner by the Russians and sent to Siberia. In his autobiographical roman a clef Political Fashions ("Politikai divatok", 1862) Mór Jókai fancied his late friend's "resurrection": ten years later Petőfi (in the novel he's named Pusztafi) returns as a shabby, declassed figure who lost his faith even in poetry. In 1990 an eccentric millionaire organised an expedition to Barguzin, where some archaeologists claimed to have unearthed Petőfi's skeleton.

Petőfi started his career as a poet with "popular situation songs", a genre to which his first published poem, A borozó ("The Wine Drinker", 1842), belongs. It is the song of a drinker praising the healing power of wine to drive away all troubles. This kind of pseudo-folk song was not unusual in Hungarian poetry of the 1840s, but Petőfi soon developed an original and fresh voice which made him stand out. He wrote many folk song-like poems on the subjects of wine, love, romantic robbers etc. Many of these early poems have become classics, for example the love poem A virágnak megtiltani nem lehet ("You Cannot Forbid the Flower", 1843), or Befordultam a konyhára ("I Turned into the Kitchen", 1843) which uses the ancient metaphor of love and fire in a playful and somewhat provocative way.

The influence of folk poetry and 19th-century populism is very significant in Petőfi's work, but other influences are also present: Petőfi drew on sources such as topoi of contemporary almanac-poetry in an inventive way, and was familiar with the works of major literary figures of his day, including Percy Bysshe Shelley, Pierre-Jean de Béranger and Heinrich Heine.

Petőfi's early poetry was often interpreted as some kind of role-playing, due to the broad range of situations and voices he created and used. Recent interpretations however call attention to the fact that in some sense all lyrical poetry can be understood as role-playing, which makes the category of "role-poems" (coined especially for Petőfi) superfluous. While using a variety of voices, Petőfi created a well-formed persona for himself: a jaunty, stubborn loner who loves wine, hates all kinds of limits and boundaries and is passionate in all he feels. In poems such as Jövendölés ("Prophecy", 1843) he imagines himself as someone who will die young after doing great things. This motif recurs in the revolutionary poetry of his later years.

The influence of contemporary almanac-poetry can be best seen in the poem cycle Cipruslombok Etelke sírjára ("Branches of Cypress for Etelke's Tomb", 1845). These sentimental poems, which are about death, grief, love, memory and loneliness were written after a love interest of Petőfi's, Etelke Csapó, died.

In the years 1844-45 Petőfi's poetry became more and more subtle and mature. New subjects appeared, such as landscape. His most influential landscape poem is Az Alföld ("The Plains"), in which he says that his homeland, the Hungarian plains are more beautiful and much dearer than the Carpathian mountains; it was to become the foundation of a long-lived fashion: that of the plains as the typical Hungarian landscape. Petőfi's poetic skills solidified and broadened. He became a master of using different kinds of voices, for example his poem A régi, jó Gvadányi ("The Good Old Gvadányi") imitates the style of József Gvadányi, a Hungarian poet who lived at the end of the 18th century.

It is interesting to note that several of Petőfi's poems were set to music by the young Friedrich Nietzsche, who composed as a hobby while studying classics at Pforta before beginning his career in philosophy. Petőfi maintained a lifelong friendship with János Arany, another significant poet of the time. Arany was the godfather of Petőfi's son Zoltán.

After the Revolution was crushed, Petőfi's writing became immensely popular, while his rebelliousness served as a role model ever since for Hungarian revolutionaries and would-be revolutionaries of every political colour. Today, streets and squares are named after him throughout Hungary and Hungarian-speaking regions of neighboring states; in Budapest alone, there are 11 Petőfi streets and 4 Petőfi squares. A national radio station (Radio Petőfi), a bridge in Budapest and a street in Sofia, Bulgaria, also bear his name, as well as the asteroid 4483 Petöfi, a member of the Hungaria family. Petőfi has a larger than life terra cotta statue near the Pest end of Erzsébet Bridge, sculpted by Miklós Izsó and Adolf Huszár. There is a Petőfi statue in nearly every Hungarian city.