Ladislaus was the second son of the future King Béla I of Hungary and his wife princess Richeza. He was born in Poland, where his father had sought refuge after his father (Ladislaus' grandfather) made an unsuccessful attempt against his cousin, Saint Stephen I, the first king of Hungary. He was named according to the Slavic traditions of his mother's kin and thus he brought the name László to yet increasing Hungarian use.

In 1048, the family moved to Hungary, where his father received as appanage one third of Hungary ("Tercia pars Regni") from his brother, King Andrew I of Hungary who had acquired the throne from King Peter after a pagan revolt. Following his accession, King Andrew I had to face the attacks of Henry III, Holy Roman Emperor, whose supremacy had been acknowledged by King Peter. King Andrew I and Duke Béla cooperated closely against the German attacks and they would preserve Hungary's independence. However, their cooperation began to loosen from 1053 when the king fathered a son, Salamon, because from that time King Andrew wanted to ensure his son's inheritance against his brother, who pursuant to the old Hungarian custom, as the oldest member of the royal family, could lay claim to the throne in case of the king's death.

In 1057, King Andrew I had Salamon crowned to ensure his accession, and Ladislaus had to participate in the coronation together with his father and his brothers, Géza and Lampert. However, Duke Béla and his sons left the country in 1059 and they returned with Polish troops in the next year. King Andrew I lost two battles against his brother and died. After his death Ladislaus' father was crowned on December 6, 1060. After his father's death on September 11, 1063, Ladislaus and his brothers agreed to accept the rule of their cousin who had come back followed by German troops, if they received their father's former duchy. However, King Salamon refused the offer and the superiority of his troops obliged the three brothers to leave Hungary. They went to Poland, and after the withdrawal of the German army, they came back to Hungary followed by troops King Bolesław II, their maternal cousin, provided them.

The parties, however, wanted to avoid civil war; therefore they accepted the mediation services of the bishops. They made an agreement on January 20, 1064, in Győr, under which Ladislaus and his brothers accepted Salamon's rule, and they received their father's former duchy, i.e., the one third of Hungary.

In the following years, Ladislaus and his brothers collaborated successfully with King Salamon. In 1068, when the Pechenegs (besenyők) had overrun the territories of Transylvania, Ladislaus, his brothers and the king went together against them and won a victory at Kerlés. A popular legend of Ladislaus connected to this battle tells of Ladislaus pursuing and overcoming a Pecheneg warrior, who had tried to abduct a Hungarian girl, thus freeing the girl.

At that time, Ladislaus married his first wife who was probably a daughter of a German count.

Starting in 1071, when Ladislaus' elder brother, Duke Géza, refused to hand over the king's share of the booty from the Byzantine Empire acquired after the occupation of Belgrade, the relationship between King Salamon and the three brothers deteriorated. Thus, in the next year, when the king lead a new campaign against Belgrade, only Duke Géza followed him. Ladislaus and Lampert were left behind because the dukes worried that the king's partisans would try to conquer their duchy during their absence.

During 1073, both King Salamon and his cousins prepared for the coming struggle. The king sent his envoys to his brother - in - law, King Henry IV of Germany, while Ladislaus and his brothers sought the help of their Polish and Czech relatives. Ladislaus went to Moravia and came back with the troops Duke Otto of Moravia, his brother-in-law, had provided him. He came just in time, because before his arrival his brother, Duke Géza had been defeated by King Salamon in the battle at Kemej on February 26, 1074. On March 14, at the Battle of Mogyoród, the three brothers won a decisive victory over King Salamon's troops, who fled to the western parts of Hungary. Géza was proclaimed king by the dukes' followers. The new king confirmed his brothers, Ladislaus and Lampert, in the possession of their duchy.

During his brother's reign, Ladislaus was his military commander, and in the autumn of 1074, he forced back King Salamon's attack against Nyitra. However, in 1076, he could not take Pozsony from King Salamon (although, according to his legends, he would overcome his cousin in single combat).

After the death of his brother on April 25, 1077, Ladislaus was proclaimed king by their partisans. He was probably crowned with the crown sent by the Emperor Michael VII to his brother, because the ancient crown was still in the possession of King Salamon.

When Ladislaus was crowned, the counties of Moson and Pozsony, were still under the rule of King Salamon, who could count on the assistance of his brother - in - law, King Henry IV of Germany. Therefore, Ladislaus sought the alliance of the German king's rivals, and in 1078, he married Adelaide, the daughter of Duke Rudolf I of Swabia, who had been proclaimed King of Germany by the emperor's opponents.

In 1079, Ladislaus took the fortress of Moson from King Salamon, but he was not able to occupy Pozsony. Afterwards, he began negotiations with his rival, who finally abdicated in his favour in 1081 in exchange for extensive landholdings. Although, the deposed king tried to plot against his cousin, Ladislaus overcame the conspiracy and had Salamon imprisoned.

Upon Ladislaus' initiative, Pope Gregory VII ordered the canonization of the first king of Hungary, Stephen I and his son, Emeric (Imre). On the occasion of the celebrations, on the feast of the Assumption, August 15, 1083, Ladislaus allowed Salamon to go free. Salamon subsequently fled to the Pechenegs. In 1085, the Pechenegs invaded the eastern territory of the kingdom, but Ladislaus defeated them. Following upon his victory, no one disputed Ladislaus' right to rule.

The terms Nobilissimus (most noble) and nobilissima familia (most noble family) have been used since the 11th century for the King of Hungary and his family, but it were then only a few, among them also Ladislaus I, which were mentioned in official documents as such.

The continuous struggles for the throne following the death of Saint Stephen I in 1038, had resulted in a confused internal situation by the time Ladislaus ascended the throne. Therefore, Ladislaus issued extremely severe decrees against criminal offenders that made provision for penalties such as mutilation, enslavement or execution for minor crimes against property or the Christian faith.

King Ladislaus took an active part in the reorganization of the Roman Catholic Church in Hungary, by the setting up of a new bishopric in Zagreb in 1094. The archbishopric of Bacs was founed by separating it from Kalocsa, and the See of Bihar was transferred to Nagyvárad, which was not entirely in line with the normative practice of the Church. Similarly, the synod of Hungarian prelates at Szabolcs in the year 1092 recognized the legitimacy of the first marriage of the members of the clegry, which was contrary to canon law.

The collapse of the German emperor in his struggle with the pope left Ladislaus free to extend his dominions towards the south and east toward the eastern Carpathians. In 1087, he sent his envoys to the court of Herman of Salm, who had been proclaimed King of Germany by the opponents of Henry IV, Holy Roman Emperor, following the death of Ladislaus' father - in - law, but after he received information on Salamon's death, he did not intervene in the internal struggles in Germany.

In 1092, Ladislaus led his armies against Prince Vasilko of Terebovlia, who had allied himself with the Cumans, and won a victory over him. In 1093, Ladislaus supported Duke Zbigniew's revolt against his father, Duke Władysław I Herman of Poland.

Following the death of King Dmitar Zvonimir on April 20, 1089, who had been married to Ladislaus' sister Ilona, the new King of Croatia, who had grown old and had never married, Stephen II, was taken out of his monastery and crowned. With his death in 1091 the Croatian ruling dynasty ended and Ladislaus proclaimed his claim to kingdom as closest living relative of Croatian ruling dynasty. Ladislaus' alternative, but much weaker claim, had been based in Byzantine legal traditions in which it is the right of an emperor's widow to choose his successor and Ilona had declared her support for him. In 1091, Hungarian troops entered Croatia. Ladislaus subjugated the neighboring country, and it was incorporated into Hungary.

However, this action provoked a counter move by the Byzantine Emperor Alexios I. He enlisted the aid of the Cumans and persuaded them to invade the eastern parts of Hungary. Upon hearing of the Cuman invasion, Ladislaus lead his armies against them and won a decisive victory over them near the river Temes. Ladislaus followed up his victory by his occupation of Szerém and Beograd, areas under Byzantine control. Emperor Alexios I, however, sent fresh nomad troops against Hungary which forced Ladislaus to leave Byzantine territory. It was probably King Ladislaus I who planted in Transylvania the Székely in order to defend the eastern parts of the Kingdom of Hungary against foreign invasions.

In the autumn of 1091, Pope Urban II sent a legate to Ladislaus' court and demanded that Ladislaus accept his supremacy over Croatia. Ladislaus refused this claim and he probably accepted the legitimacy of Antipope Clement III, who had been elected by the followers of the Holy Roman Emperor.

The last years of Ladislaus' reign were characterized by the strained relations with his two nephews, Coloman and Álmos. As Ladislaus did not have any sons, his two nephews, the sons of King Géza I, could expect to inherit the throne. Ladislaus preferred the younger nephew, Álmos, whom he had named king of Croatia after conquering the country. Coloman did not give up his claims to the throne, and in 1095, he left for Poland.

Ladislaus was preparing a campaign against Duke Břetislav II of Bohemia in order to help his sister's sons, Dukes Svatopluk and Otto II of Moravia, when he was informed that Coloman had come back to Hungary in the company of Polish troops. The elderly king, upon hearing the news, died suddenly.

He was buried in the Abbey of Somogyvár which he had founded in 1091.