Carlos I became King on 19 October 1889. Colonial treaties with the United Kingdom of Great Britain and Ireland (one signed in August 1890 that defined their African borders along the Zambezi and Congo rivers and another signed on 14 October 1899, that confirmed colonial treaties of the 17th century) stabilized the situation in Africa. These agreements were however unpopular in Portugal where they were seen as being to the disadvantage of the country.
Domestically, Portugal was twice declared bankrupt - on 14 June 1892, and again on 10 May 1902 - causing industrial disturbances, socialist and republican antagonism and press criticism of the monarchy. Carlos responded by appointing João Franco as prime minister and subsequently accepting parliament's dissolution.
As a patron of science and the arts, King Carlos took an active part in the celebration of the 500th anniversary (in 1894) of the birth of Prince Henry the Navigator. The following year he decorated the famous Portuguese poet João de Deus in a ceremony in Lisbon. Carlos took a personal interest in deep-sea and maritime exploration, publishing an account of his own studies in this area.