Frank Lawless took part in the 1916 Easter Rising, being second in command under Thomas Ashe in the fight at Ashbourne, County Meath. Two of his sons were also combatants on that occasion. As a result he was condemned to death, but the sentence was commuted to ten years penal servitude. He was imprisoned at Lewes with Harry Boland. He was released in the general amnesty of 1917. He was again arrested in connection with the "German Plot" and was confined in Usk prison. He was paroled to permit him to take part in the 1918 election, was present at the declaration at Balbriggan but returned to Usk prison on the same day. After his release from Usk he was interned in Ballykinlar Camp. Ashe was Frank Lawless's commanding officer, when the latter served with 4th section, Cork no.1 brigade during the war of independence. Lawless took an active part in the battle of Ashbourne, being battalion Quartermaster. A victory for the Irish volunteers, who defeated a superior number of RIC at the crossroads of Ballyannan on the Friday of Easter Week. Coming from the south in a reserve position from behind a hedgerow, his men were fired on by their own 1st section by mistake. But many of the police lay dead in the ditches, and other surrendered, thanks to timely planning and courageous leadership from Mulcahy and Ashe respectively. It was a wider attempt to raise rebellion outside Dublin.
Like the other Sinn Féin members, Lawless did not take his seat at Westminster but took part in the revolutionary First Dáil. He was re-elected to the Second Dáil, 1921–22 for the new Dublin County constituency. He was one of the majority of 64–57 who voted in favour of ratification of the Anglo-Irish Treaty in the critical debate of 7 January 1922.
He died three months later at the age of 51 from injuries received when the pony trap in which he was riding was accidentally upset. He was buried with full military honours at Killossery Cemetery, Rolestown. He was married with six sons and five daughters.