Allsopp’s father was enrolled a burgess of Nottingham, where he was a draper, in 1760. Lewis became an attorney in King’s bench, 21 June 1802. His practice at Old Pavement, Nottingham, was continued after his death by the partners of his later years, George Freeth and James Parke. In March 1807, after he had been elected a junior common councilman of Nottingham, he was at first refused his seat because he would not take an oath to keep secret the acts and proceedings of common hall. By September 1807 he was on the finance committee, and afterwards active in many municipal projects such as sewage and gas lighting.
His coming into Parliament was somewhat fortuitous. His interest in politics was intermittent, to judge by his voting behaviour at Nottingham, where he did not vote in 1803 and 1812, but generally supported the Blues and was active on behalf of Arkwright in 1812 and Assheton Smith in 1818. Before the election of 1818 he offered himself on the Hallet interest to the electors of Camelford, but withdrew on the day of the election; when that election was declared void and one of the defeated candidates died, Allsopp replaced him successfully.
