He began teaching biology and lecturing in science at King's College London but was unable to advance due to his atheism and avowed leftist views. He then joined the National Secular Society and was likely the lover of Annie Besant, one of the society's leaders. He wrote and lectured for the society but again was forced to leave after becoming a Marxist; an anathema to Charles Bradlaugh, the society's founder.
In 1884 he became the partner of Eleanor Marx, the daughter of Karl Marx. This was despite being still married to Isabel Frank, albeit separated, a situation that continued until her death in 1892. 1884 also saw him and Eleanor being elected to the executive council of the Social Democratic Federation although they left shortly after with others to form the Socialist League.
During his time in the Socialist League he wrote and translated socialist texts but he was unpopular in the movement, both for running up debts and for his relationship with Eleanor which gave him an assumed position at the heart of Marxism. His obituary in Reynolds's Paper referred to his Quasimodo like appearance and "remarkable fascination for women", as well as an oblique reference to "the tales he left behind him" on lecturing tours of the provinces.
In 1897 he left Eleanor and on 8 June that year secretly married an actress, Eva Frye, but returned to Eleanor when he was struck down with kidney disease. After nursing him for some time, Eleanor committed suicide mainly due to his infidelity. Aveling died four months later in Battersea, being cremated in Woking, Surrey, three days later. Despite his prominence as a member of the fledgling British Marxist movement, no representatives of the Socialist or Labour movements were present at the funeral as it was widely known that he was responsible for Eleanor Marx's suicide. He had numerous relationships with women, but he is not known to have fathered any children.