A native of Bucharest, he was born out of wedlock to Ion Luca Caragiale and Maria Constantinescu, an unmarried former Town Hall employee who was 21 at the time. Living his first years at his mother's house on Frumoasă Street, near Calea Victoriei (until the building was sold), Mateiu had a half-sister, his mother's daughter from another extra-conjugal affair. In 1889, almost a year after separating from his concubine, his father married Alexandrina Burelly, bringing Mateiu into his new family. In following years, he was progressively estranged from his father, and, according to Ecaterina, the youngest of Ion Luca Caragiale and Burelly's children, "Mateiu alone confronted father and contradicted him systematically."