In 1868, Davitt was given the role of IRB organising secretary and arms agent for England and Scotland. This new work required him to resign from his post office employment, and travel around Irish communities across Britain, using his role to raise funds and support for the purchase and procurement of guns for the movement. Unbeknownst to Davitt, he had come under the notice of the police following his role in the Chester Castle episode. Following the gathering of intelligence on his activities by police, Davitt was arrested on 14 May 1870 at Paddington Railway Station, London, while waiting to meet John Wilson, a gunsmith (who was also arrested).
Both men were found guilty of arms trafficking to support a Fenian uprising, and were sentenced to penal servitude, with Davitt sentenced to fifteen years. At the age of twenty-four, he began a prison sentence which would last seven and a half years, beginning his term in Millbank penitentiary in Westminster. At Millbank, he was kept in solitary confinement, and was given the job of oakum picking. This was tough, mindless work, where prisoners were given old rope and tasked with unwinding the rope into single strands, which they then had to unroll further into smaller fibres. The finished fibres would have been mixed with grease or tar to make a caulking material for filling in gaps between planks on ship timbers. For at least ten hours of each day, Davitt would have struggled with his task. With only one arm to assist him, he had to use his mouth to help with the work.
After ten months at Millbank, Davitt was moved to Dartmoor Prison in Devon. Here the conditions for prisoners were much worse, where they were kept in airless, narrow cells, and given rotten and cockroach infested food as their rations. Michael was first given the job of stone-breaking, and when his one hand became too blistered to continue in this task, he was moved onto work in a gang hauling heavy carts around the prison. He was returned to stone-breaking once again when the harness he wore for the cart injured the stump of his arm.
Michael discovered the power of his writing while in prison. He found a way to smuggle letters out of Dartmoor, which alerted the public about his experiences as a political prisoner, and which were released to the press in both Britain and Ireland. The publicity from his letters helped to strengthen the case of those who were working for his release, and Davitt was released at the age of thirty-one under a conditional licence or a ticket-of-leave on 19th December 1877. This was a provisional release, which meant that he could be returned to prison at any time. The campaign for his release had given Davitt a new public profile, and he threw himself back into his activities with the Fenians. His formative experiences in prison let him with a long-lasting desire to work for prison reform, and for the improvement of prisoner welfare.