After graduating in 1845 Lassalle moved to Paris. On his return to Germany he became friends with Countess Sophie von Hatzfeldt. At the time she was involved in a protracted legal case against her husband. Lassalle provided her with valuable support and when she won the case she paid him an annual income for the rest of his life.
Lassalle held strong political views and he became involved in the German Revolutions of 1848. Lassalle was eventually arrested and charged with inciting armed opposition to the state and resistance against public officials. Found guilty of the second charge he was sentenced to six months in prison. On his release he was deported from Germany.
With the help of Alexander von Humboldt Lassalle was given permission to live in Berlin in 1859. In return, he promised not to be active in politics. Over the next few years he wrote several books on legal matters. This included The System of Acquired Rights (1861). However, a pamphlet he published in 1862 was seized by the authorities. He was arrested and was briefly imprisoned in 1863.
Eduard Bernstein argued that at this time: "Lassalle was in his thirty-seventh year, in the full force of his physical and mental development. He had already lived a strenuous life; he had made himself a name politically and scientifically – both, it is true, within certain limited circles; he was in relations with the most prominent representatives of literature and art; he had ample means and influential friends. In a word, according to ordinary notions, the Committee, composed of hitherto quite unknown men, representing a still embryonic movement, could offer him nothing he did not already possess. Nevertheless, he entered into their wishes with the utmost readiness, and took the initial steps for giving the movement that direction which best accorded with his own views and aims."
Lassalle joined the Communist League, where he met fellow members, Karl Marx, Friedrich Engels and Wilhelm Liebknecht. Lassalle wrote several pamphlets that attempted to popularize the ideas of Marx. One of his converts was August Bebel, the future leader of the socialist movement. He wrote in his autobiography: "The open letter of Lassalle did not make at all such apt impression upon the world of labor as had been expected, in the first place, by Lassalle himself; in the second place, by the small circle of his followers. For my part, I distributed about two dozen copies in the Industrial Educational Club, in order to give the other side a chance. That the letter should have made so little impression upon the majority of the laborers in the movement of that time, may seem inexplicable today to some people. But it was quite natural. Not merely the economic, but also the political conditions were still very backward."
Karl Marx was highly critical of Lassalle's interpretation of his work. He also disapproved of Lassalle when he established the General German Workers' Association (ADAV) in May, 1863. The main objective of the organisation was to win universal suffrage through peaceful and legal means. As a result, most members of the Communist League refused to join the organisation.
Élie Halévy argued in The Era of Tyrannies: Essays on Socialism and War (1938): "Lassalle was the first man in Germany, the first in Europe, who succeeded in organising a party of socialist action. Yet he viewed the emerging bourgeois parties as more inimical to the working class than the aristocracy' and hence he supported universal manhood suffrage at a time when the liberals preferred a limited, property-based suffrage which excluded the working class and enhanced the middle classes. This created a strange alliance between Lassalle and Bismarck."
Otto von Bismarck later admitted that he had been in contact with Lassalle: "I saw him perhaps three or four times altogether... He attracted me as an individual. He was one of the most intelligent and likable men I had ever come across. He was very ambitious and by no means a republican. He was very much a nationalist and a monarchist. His ideal was the German Empire, and here was our point of contact. As I have said he was ambitious, on a large scale, and there is perhaps room for doubt as to whether, in his eyes, the German Empire ultimately entailed the Hohenzollern or the Lassalle dynasty.... Our talks lasted for hours and I was always sorry when they came to an end."
Lassalle became romantically involved with a young woman, Helene von Dönniges, and in the summer of 1864 they decided to marry. Her father objected to the relationship and she eventually agreed to marry a nobleman named Bajor von Racowitza. Lassalle sent a challenge to duel to Racowitza. A duel took place on the morning of 28th August 1864. Lassalle was mortally wounded, and he died on 31st August.