Born in Gießen as the son of Katharina Elisabeth Henrietta (née Hirsch) and Hessian public official Ludwig Christian Liebknecht, Liebknecht grew up with relatives after the death of his parents in 1832. From 1832 to 1842, he went to school at the Gymnasium of Gießen, then began studying philology, theology and philosophy in Gießen, Berlin and Marburg. After a brush with the authorities as a result of participating in student radicalism, Liebknecht decided to emigrate to the USA.

While on the train, quite by chance, he met the headmaster of a progressive school in Zurich, Switzerland, and Liebknecht impulsively decided to accept an offer to be an unpaid teacher at that school. Thus he found himself in Switzerland in 1847 as a civil war erupted in that country. He reported these events for a German newspaper, the "Mannheimer Abendzeitung", beginning a career in journalism that he would pursue for the following five decades.

When revolution erupted in Paris in February 1848, Liebknecht hurried to the scene. He arrived too late to do much in Paris, but he did join a legion that was heading for Germany to instigate revolution there. In the course of that poorly planned expedition, he was arrested in Baden and charged with treason. On the eve of his trial, revolution erupted once more, and a mob secured his release. He then became a member of the Badische Volkswehr and an adjutant of Gustav von Struve and fought in the ill fated Reichverfassungskämpfe ("federal constitution wars"). After the revolutionaries' defeat, he escaped to Switzerland and became a leading member of the Genfer Arbeiterverein (Worker's Association of Geneva), where he met Friedrich Engels.

In 1850, Liebknecht was arrested for his initiatives to unite Switzerland's German workers' associations, and he was banished from the country. With few options available, like many veterans of the recently failed revolution, he moved into exile to London, where he stayed from 1850 to 1862. There he became a member of the Communist League. During these years, he developed a lifelong friendship and collaboration with Karl Marx. In 1862, after an amnesty for the participants in the revolution of 1848 / 1849, he returned to Germany and became a member of Ferdinand Lassalle's ADAV (Allgemeiner Deutscher Arbeiterverein, "General German Workers' Association"), the precursor of the SPD.

From 1864 to 1865, Liebknecht also worked on the magazine "Der Social - Demokrat" ("the Social Democrat") published by Jean Baptista von Schweitzer; however, he soon found himself in disagreement with the paper's friendly position toward Prussia and its new Minister - President, Otto von Bismarck. So Liebknecht left the editorial staff and was forced to leave the ADAV due to pressure from Schweitzer. After being evicted from Berlin by government authorities, Liebknecht moved to Leipzig, where he met August Bebel, with whom he founded the Sächsische Volkspartei ("Saxon People's Party") in 1867 and the SDAP (Sozialdemokratische Arbeiterpartei Deutschlands, in 1869 in Eisenach. In these years, he was elected to the national legislature, where he conducted a determined but futile opposition to Bismarck's policies. Liebknecht was also the editor of the party organ, "Der Volksstaat" ("the People's State").

In 1870, when Bismarck provoked France into war, Liebknecht used his newspaper to agitate against the war, calling on working men on both sides of the border to unite in overthrowing the ruling class. As a result, he and Bebel were arrested and charged with high treason. It is worth noting that Liebknecht opposed the war regardless of which side started it. His call for revolutionary opposition to the war directly contradicts what his party, the SPD, would do in 1914 when World War I erupted. At that time, with Liebknecht long dead, his successors opted to back the German cause in the war.

In 1872, both Liebknecht and Bebel were convicted and sentenced to two years of Festungshaft ("imprisonment in a fortress"). This was one of sixteen times that Liebknecht's politics resulted in his conviction and incarceration.

After being re-elected into the Reichstag in 1874, Liebknecht played a key role in the merger of the SDAP and Lassalle's ADAV into the SAPD (Sozialistische Arbeiterpartei Deutschlands, "Socialist Workers' Party of Germany") in Gotha in 1875. He also became publisher of the newly founded party organ "Vorwärts" ("forward"), arguing for the integration of Marxist theories into the SAPD's program in his articles.

From 1878 to 1890, the German government outlawed Liebknecht's party, but the terms of the law allowed the party to participate in elections and its elected delegates to participate in the Reichstag. Liebknecht used his position as a Reichstag member to criticize the political situation, and he combated the tendencies in his own party toward anarchism on the one hand and accommodation with Bismarck on the other. Maintaining a radical and unified stance, the SPD emerged from outlawry in 1890 with 20% of the vote in the Reichstag election.

In 1891, Liebknecht became editor - in - chief of "Vorwärts" and one of the originators of the SPD's new Marxist inspired party platform. Throughout that decade, he continued to serve in the Reichstag and to appear at political conventions of the SPD as a prominent referent. Despite his advanced age, he also was the most prominent leader of the Second Socialist International.

Liebknecht died on 7 August 1900 in Charlottenburg, a suburb of Berlin. 50,000 joined his funeral procession.