Admitted on 5 February 1601 as a student of Lincoln's Inn, where his father and grandfather had both attained legal honours, he was called to the bar 25 January 1614, became governor of the society in 1630, and was two years later nominated Autumn Reader.
On 7 February 1633 Atkyns appeared before the Star Chamber as counsel for William Prynne, charged with libelling the queen Henrietta Maria in his Histriomastix, and defended his client's character from his personal acquaintance with him.
Serjeant Atkyns should be made justice of the King's Bench’, and on 28 October 1645, despairing of any settlement with the crown, they created him, by their own order, baron of the exchequer.
[1] After the king's death, Atkyns, according to Foss, refused to accept a commission from the provisional council of state continuing him in his office, but on 9 December 1650 he was nominated, without protest on his part, one of the judges to try disturbers of the peace in the eastern counties, and was consulted by Oliver Cromwell on legal business.
The only instance in which Atkyns openly refused to act with the Commonwealth authorities was in June 1654 at the trial, by special commission, of John Gerard and others for conspiracy to murder Cromwell.