Ibrăileanu was born into a family of Armenian origin, in Târgu Frumos, Iaşi County. During the 1890s, he was attracted to Socialism, and began a collaboration with the left-wing press - periodicals such as Munca and Adevărul. He adopted part of the themes and goals expressed by the defunct Junimea, merging them with the ideas of Marxist thinker Constantin Dobrogeanu-Gherea, into a new form of Romanian populism (Poporanism), making it the main attribute of the magazine he led. He is remembered as the first mentor to such diverse figures as Mihail Sadoveanu, Ion Agârbiceanu, Ionel Teodoreanu, Gala Galaction, Octavian Goga, George Topîrceanu or Tudor Arghezi.
In his first major essay (1908), Spiritul critic în cultura românească (roughly: "Selective Attitudes in Romanian Culture"), Ibrăileanu analysed the trends in Romanian literature from cca. 1840 to cca. 1880, trying to establish what had been the characteristics of original works. This is the first draft of his theory of selection, through which he determined the relationship between social context and artists' subjectivism (using it to explain why original artists had been ignored in favor of conformist ones of lesser talent). His thesis found its first major critic in modernist figure Eugen Lovinescu.
He expanded the idea in works of literary criticism that are still influential: in 1909 - Scriitori şi curente ("Writers and Trends"); in 1912 - Opera literară a d-lui Vlahuţă ("Mr. Vlahuţă's Literary Works"), a doctorate thesis that featured one of Ibrăileanu's most quoted chapters, Literatura şi societatea ("Literature and Society"); in 1930 - Studii literare ("Literary Studies"), containing his other major writing, Creaţie şi analiză ("Creation and Analysis"). He also authored a volume of aphorisms (1930), as well as a novel - Adela (1933).
Ibrăileanu died in Bucharest. Denied admission to the Romanian Academy throughout his life, Garabet Ibrăileanu received posthumous membership in 1948.