Preston began the brighter and Paul Gallagher opened the scoring on 19 minutes but Italy under-20 international De Girolamo struck midway through the second half for the Blades.
Former United striker Gallagher, who scored twice to help defeat Norwich in the last round, fired home a brilliant 30-yard free-kick to give the hosts the lead.
North End then had a great chance to double their advantage on 40 minutes as Kyel Reid's cross from the left picked out Neil Kilkenny who opted to knock across goal rather than shoot from six yards.
United, who face Tottenham in the second leg of the Capital One Cup semi-final on Wednesday, almost levelled after 53 minutes but De Girolamo saw his effort cleared off the line by Calum Woods.
But Nigel Clough's side found an equaliser on 68 minutes as substitute Jamie Murphy cut back for De Girolamo to strike home left-footed from eight yards.