It was an influential period in his life. He saw first hand how unjust the system was, (and the British Army was there to enforce this). He developed a deep hatred for the British Army that lasted his entire life. When he heard the regiment was being transferred to India, he deserted the army.

Connolly had another reason for not wanting to go to India: a young woman by the name of Lillie Reynolds. Lillie moved to Scotland with James after he left the Army and they married in April 1890. They settled in Edinburgh. There, Connolly began to get involved in the Socialist Movement but with a young family to support, he needed a way to provide for them.

He briefly established a cobblers shop in 1895, but this failed after a few months as his shoe-mending skills were insufficient. He was also strongly active with the socialist movement at the time, and he prioritized this over his own work.

He became secretary of the Scottish Socialist Federation. At the time his brother John was secretary; after John spoke at a rally in favour of the eight hour day, however, he was fired from his job with the Edinburgh Corporation, so while he looked for work, James took over as secretary. During this time, Connolly became involved with the Independent Labour Party which Keir Hardy had formed in 1893.

Sometime during this period, he took up the study of, and advocated the use of, the neutral international language, Esperanto.

By 1892 he was involved in the Scottish Socialist Foundation, acting as its secretary from 1895. Two months after the birth of his third daughter, word came to Connolly that the Dublin Socialist Club was looking for a full-time secretary, a job that offered a salary of a pound a week. Connolly and his family moved to Dublin, where he took up the position. While active as a socialist in Great Britain, Connolly was the founding editor of The Socialist newspaper and was among the founders of the Socialist Labour Party which split from the Social Democratic Federation in 1903. While in America he was a member of the Socialist Labour Party Of America (1906), the Socialist Party of America (1909) and the Industrial Workers of the World, and founded the Irish Socialist Federation in New York, 1907. He famously had a chapter of his 1910 book "Labour in Irish History" entitled "A chapter of horrors: Daniel O'Connell and the working class." critical of the achiever of Catholic Emancipation 60 years earlier.

On his return to Ireland he was right hand man to James Larkin in the The Irish Transport and General Workers Union. He stood twice for the Wood Quay ward of Dublin Corporation but was unsuccessful. His name, and those of his family, appears in the 1911 Census of Ireland - his occupation is listed as "National Organiser Socialist Party". In 1913, in response to the Lockout, he, along with an ex-British officer, Jack White, founded the Irish Citizens Army (ICA), an armed and well-trained body of labour men whose aim was to defend workers and strikers, particularly from the frequent brutality of the Dublin Police. Though they only numbered about 250 at most, their goal soon became the establishment of an independent and socialist Irish nation. He founded the Irish Labour Party as the political wing of the Irish Trade Union Congress in 1912 and was a member of its National Executive. Around this time he met Winifred Carney in Belfast, who became his secretary.

Connolly was sentenced to death by firing squad for his part in the rising. On 12 May 1916 he was transported by military ambulance to Kilmainham Gaol, carried to a prison courtyard on a stretcher, tied to a chair and shot. His body (along with those of the other rebels) was put in a mass grave without a coffin. The executions of the rebels deeply angered the majority of the Irish population, most of whom had shown no support during the rebellion. It was Connolly's execution, however, that caused the most controversy. Historians have pointed to the manner of execution of Connolly and similar rebels, along with their actions, as being factors that caused public awareness of their desires and goals and gathered support for the movements that they had died fighting for.

Connolly Station one of the two main railway stations in Dublin, and Connolly Hospital Blanchardstown, are named in his honour.

In a 2002, BBC television production, 100 Greatest Britons where the British public were asked to register their vote, Connolly was voted in 64th place.

James Connolly was born in an Irish slum in Edinburgh in 1868. His parents had emigrated to Scotland from Monaghan and settled in the Cowgate, an Irish ghetto where thousands of Irish settled.

He was born in St Patrick's Roman Catholic parish, which was known as "Little Ireland". His father and grandfathers were labourers. He had an education up to the age of about ten in the local Catholic primary school. He then left and worked in labouring jobs. Because of the economic difficulties he was having,like his eldest brother John, he joined the British Army.

He enlisted in the Army at age 14, falsifying his age and giving his name as Reid, as his brother John had done. He served in Ireland with the Army for nearly seven years. It was a very turbulent period in rural Ireland.He would later become involved in the land issue.