A son of Vladimir the Great, the first Christian Prince of Novgorod, Yaroslav acted as vice-regent of Novgorod at the time of his father's death in 1015. Subsequently, his eldest surviving brother, Sviatopolk I of Kiev, killed three of his other brothers and seized power in Kiev. Yaroslav, with the active support of the Novgorodians and the help of Varangian mercenaries, defeated Svyatopolk and became the Grand Prince of Kiev in 1019. Under Yaroslav the codification of legal customs and princely enactments was begun, and this work served as the basis for a law code called the Russkaya Pravda ("Rus Truth "). During his lengthy reign, Kievan Rus' reached the zenith of its cultural flowering and military power.
The early years of Yaroslav's life are shrouded in mystery. He was one of the numerous sons of Vladimir the Great, presumably his second by Rogneda of Polotsk, although his actual age (as stated in the Primary Chronicle and corroborated by the examination of his skeleton in the 1930s) would place him among the youngest children of Vladimir. It has been suggested that he was a child begotten out of wedlock after Vladimir's divorce from Rogneda and marriage to Anna Porphyrogenita, or even that he was a child of Anna Porphyrogenita herself. Yaroslav figures prominently in the Norse sagas under the name Jarisleif the Lame; his legendary lameness (probably resulting from an arrow wound) was corroborated by the scientists who examined his remains.
In his youth, Yaroslav was sent by his father to rule the northern lands around Rostov but was transferred to Veliky Novgorod, as befitted a senior heir to the throne, in 1010. While living there, he founded the town of Yaroslavl (literally, "Yaroslav's") on the Volga River. His relations with his father were apparently strained, and grew only worse on the news that Vladimir bequeathed the Kievan throne to his younger son, Boris. In 1014 Yaroslav refused to pay tribute to Kiev and only Vladimir's death, in July 1015, prevented a war.
During the next four years Yaroslav waged a complicated and bloody war for Kiev against his half-brother Sviatopolk I of Kiev, who was supported by his father-in-law, Duke Bolesław I Chrobry of Poland. During the course of this struggle, several other brothers (Boris, Gleb, and Svyatoslav) were brutally murdered. The Primary Chronicle accused Svyatopolk of planning those murders, while the saga Eymundar þáttr hrings is often interpreted as recounting the story of Boris' assassination by the Varangians in the service of Yaroslav. However, the victim's name is given there as Burizaf, which is also a name of Boleslaus I in the Scandinavian sources. It is thus possible that the Saga tells the story of Yaroslav's struggle against Svyatopolk (whose troops were commanded by the Polish duke), and not against Boris.
Yaroslav defeated Svyatopolk in their first battle, in 1016, and Svyatopolk fled to Poland. But Svyatopolk returned in 1018 with Polish troops furnished by his father-in-law, seized Kiev and pushed Yaroslav back into Novgorod. Yaroslav at last prevailed over Svyatopolk, and in 1019 firmly established his rule over Kiev. One of his first actions as a grand prince was to confer on the loyal Novgorodians (who had helped him to gain the Kievan throne), numerous freedoms and privileges. Thus, the foundation of the Novgorod Republic was laid. For their part, the Novgorodians respected Yaroslav more than they did other Kievan princes; and the princely residence in their city, next to the marketplace (and where the veche often convened) was named Yaroslav's Court after him. It probably was during this period that Yaroslav promulgated the first code of laws in the lands of the East Slavs, the Russkaya Pravda.
Leaving aside the legitimacy of Yaroslav's claims to the Kievan throne and his postulated guilt in the murder of his brothers, Nestor the Chronicler and later Russian historians often presented him as a model of virtue, styling him "the Wise". A less appealing side of his personality is revealed by his having imprisoned his youngest brother Sudislav for life. Yet another brother, Mstislav of Chernigov, whose distant realm bordered the North Caucasus and the Black Sea, hastened to Kiev and, despite reinforcements led by Yaroslav's brother-in-law King Anund Jacob of Sweden (as Jakun - "blind and dressed in a gold suit"), inflicted a heavy defeat on Yaroslav in 1024. Yaroslav and Mstislav then divided Kievan Rus' between them: the area stretching left from the Dnieper River, with the capital at Chernihiv, was ceded to Mstislav until his death in 1036.
In his foreign policy, Yaroslav relied on the Scandinavian alliance and attempted to weaken the Byzantine influence on Kiev. In 1030, he reconquered Red Ruthenia from the Poles and concluded an alliance with King Casimir I the Restorer, sealed by the latter's marriage to Yaroslav's sister, Maria. In another successful military raid the same year, he captured Tartu, Estonia and renamed it Yuryev (named after Yury, Yaroslav's patron saint) and forced the surrounding Ugandi County to pay annual tribute.