Dmowski was born on 9 August 1864 in Warsaw's Kamionek district, in Congress Poland (Vistula Land), then part of the Russian Empire. His father was a road construction worker and later an entrepreneur. Dmowski attended schools in Warsaw, studying biology and zoology at Warsaw University, from which he graduated in 1891. As a student he became active in the Polish Youth Association "Zet" (Związek Młodzieży Polskiej "Zet"), where he was active in opposing socialist activists. He also organized a student street demonstration on the 100th anniversary of the Polish Constitution of 3 May 1791. For this he was imprisoned by Russian Imperial authorities for five months in the Warsaw Citadel. After 1890 he was also developing as a writer and publicist, publishing political and literary criticism in Głos, where he became close friends with Jan Ludwik Popławski, who would be his mentor.
In April 1893 Dmowski co-founded the National League (Liga Narodowa), and became its first leader. In November that year he was sentenced to exile from the Vistula Land. Dmowski went to Jelgava, and soon afterward in early 1895 to Lemberg, Austria-Hungary (modern Lviv, Ukraine; known as Lwów to the Poles), where together with Popławski he began to publish a new magazine,
Przegląd Wszechpolski (All-Polish Review). In 1897, he co-founded the National-Democratic Party (Stronnictwo Narodowo-Demokratyczne or "Endecja"). The Endecja was to serve as a political party, a lobby group and an underground organization that would unite Poles who espoused Dmowski's views into a disciplined and committed political group. In 1899, Dmowski founded the Society for National Education as an ancillary group. From 1898 to 1900, he resided in both France and Britain, and travelled to Brazil. In 1901 he took up residence in Kraków, then part of the Austrian partition of Poland. In 1903 he published a book,
Myśli nowoczesnego Polaka (Thoughts of a Modern Pole), one of the first if not the first nationalist manifesto in European history.
Dmowski opposed revolutionary means of fighting, preferring political struggle, and aimed for independence through increased autonomy. After the outbreak of the Russo-Japanese War, Dmowski met with Colonel Akashi Motojiro, the Japanese military attache in Sweden and spy-master for Japanese intelligence activities, in Kraków in March 1904. Although reluctant to collaborate with the Japanese, Dmowski agreed to Akashi's proposal that Polish soldiers in Manchuria might be encouraged to defect to the Imperial Japanese Army. He travelled to Tokyo to work out the details, and at the same time made a successful effort to prevent the Japanese from aiding a rival Polish political activist, Józef Piłsudski, who wanted assistance for a planned insurrection in Poland, an aspiration which Dmowski felt would be doomed to failure.
In 1905, Dmowski moved to Warsaw, back in the Russian partition of Poland, where he continued to play a growing role in the Endecja faction. During the Russian Revolution of 1905, Dmowski favoured co-operation with the Imperial Russian authorities and welcomed Nicholas II's October Manifesto of 1905 as a stepping stone on the road towards renewed Polish autonomy. During the revolt in Łódź in June 1905, the Endeks, acting under Dmowski's orders, opposed the uprising led by Piłsudski's Polish Socialist Party (PPS). During the course of the "June Days", as the Łódź uprising is known, a miniature civil war raged between Endecja and the PPS.
As a result of the elections to the First Duma (legislative assembly in the late Russian Empire), which were boycotted by the PPS, the National Democrats won 34 of the 55 seats allotted to Poland. Dmowski himself was elected a deputy to the Second and Third Dumas (beginning on 27 February 1907) and was president of the Polish caucus within it. He was seen as a conservative, and despite being a Polish caucus leader, he often had more influence on the Russian than the Polish deputies.
Over time, Dmowski became more receptive to Russian overtures, particularly neoslavism, warming up to the idea that Poland and Russia may have a common future, particularly due to Germany being their common enemy. In light of what he regarded as Russian cultural inferiority, Dmowski felt that a strong Russia was more acceptable than a strong Germany. In Dmowski's view, the Russian policy of Russification would not succeed in subjugating the Poles, while the Germans would be far more successful with their Germanisation policies. He explained those views in his book Niemcy, Rosja i kwestia Polska (Germany, Russia and the Polish Cause), published in 1908.
This was not a universally popular attitude, and in 1909 Dmowski resigned his deputy mandate to focus on an internal political struggle within Endecja. He lost the election to the Fourth Duma in 1912 to a socialist politician, Eugeniusz Jagiełło from the Polish Socialist Party – Left, who won with the support of the Jewish vote. Dmowski viewed this as a personal insult; in exchange he organized a successful boycott of Jewish businesses throughout much of Poland.