Until 1152 he ruled with Melisende, but in that year, feeling himself able to rule alone, he broke away and, after a complicated series of manoeuvres (including the king crowning himself when the Patriarch of Jerusalem refused to carry out a coronation), ended up forcing her into a nunnery. Later, reconciled, he used his mother's advice to help him rule well, and she was his choice as regent when campaigning took him out of the city.
He was a competent military leader, defending the kingdom against Muslim attacks, although he was unable to achieve anything when the Second Crusade arrived in 1148, and was instead persuaded to attack Damascus (then friendly to the Christians), which achieved nothing but demonstrating the poor faith of the Crusaders. It was during his reign that, with the capture of Ascalon in 1153, Outremer reached its greatest extent. However, he was faced by the growing power of Nuradin who, in the following year, captured Damascus. Thereafter much of Baldwin's energy had to be directed towards countering Nuradin's advances and keeping the peace with his relatives in Tripoli and Antioch (which were effectively independent states, albeit under Crusader rule). On his return from one trip north, in 1162, Baldwin fell ill and weakened and died.
Diplomatically he endeavoured to force links with Byzantium, marrying Theodora, the niece of Emperor Manuel, in 1158, when she was 13. Two years later Manuel married Baldwin's cousin, Maria, although this was in part as a means of acquiring more power within Antioch.
Since Baldwin had never had children, on his death he was succeeded by his brother Amalric. Theodora retired to Acre (she had never been allowed to play any role in the kingdom).
Baldwin was the first Crusader king who was born within Outremer, and was generally reckoned to have been a good ruler by his contemporaries.