At the turn of the 19th century a number of novelties were introduced in Ljubljana, such as waterworks (1890), public electricity supply and a modern sewage system (1898), tram services (1901) and the first cinema (1907). The city was affected by World War I only indirectly and from afar. In 1918, after the disintegration of the Austro-Hungarian empire, Ljubljana became an administrative, political and cultural centre of Slovenia as part of the Kingdom of Serbs, Croats and Slovenians. During the existence of the Kingdom, the city saw the foundation of the University of Ljubljana (1919), the National Gallery (1918) and the Academy of Sciences and Arts (1938).

In the mid-1930s, it had a population of more than 80,000 inhabitants. Especially in the Bežigrad district new residential quarters were built on the principles of modernist functionalist architecture. In the period between the two World Wars, it was architect Jože Plečnik who left a strong mark on the appearance of Ljubljana, achieving through his work a harmony between the Romance Baroque and the Germanic Art Nouveau styles. His personal mark on the period was so strong that the city's architecture of his era has often been referred to as Plečnik's Ljubljana.

During World War II, the city was occupied first by the Italian and later by the German army. In order to break the strong resistance movement, in 1942 the occupiers surrounded Ljubljana with a 30-kilometre barb wire fence, whose course is now the course of a walking path around the city. After the war Ljubljana became the capital of Slovenia, one of the six republics constituting socialist Yugoslavia. The post-war period was a time of rapid economic development, which attracted numerous immigrants and resulted in the city's expansion.

In the plebiscite held on 23 December 1990, the citizens of Slovenia voted for an autonomous and independent state, which was declared on 25 June 1991, with Ljubljana as its capital. In May 2004 Slovenia entered the EU.