There is a Harriet Mansfield baptised 8 or 9 Oct 1823 at Tenterden, parents James and Elizabeth.
And a Harriet Elizabeth Mansfield baptised 13 Feb 1825 at Brighton, parents James and Elizabeth.
Also there is a Harriet Mansfield age 19 admitted to Castle Street workhouse in Westminster 12 Apr 1845, "With Child of a Bastard; Settlement to be enquired into".
https://www.ancestry.co.uk/imageview...ce&pId=2158014
There are a few more workhouse admissions and discharges for her, and some criminal records which could be her.