Anita Gaspar netted an extra-time winner to fire the Ewe Rams into the Second Round of the League Cup.
Jessica Gillin had put the home side ahead in the first half but it was cancelled out by Ella Hilliard's 92nd minute equaliser.
Derby had early shots inside the first two minutes of the match for Keria Barry and Emelia Wilson, the latter saved by Mollie Finch.
The game settled down into a period of possession for both sides. A dangerous Chorley free kick was met with a shot on goal that Sarah Morgan had to save.
Isabel Catlow made a great block to deny Becky McGrother's shot heading on target.
Kira Rai had a good chance when she used her burst of pace to beat a defender and advance into the penalty area but shot into the side netting.
Gillin gave the hosts a surprise lead when she lobbed Morgan to open the scoring on 38 minutes.
On the stroke of half-time Becky McGrother's strike came close to sailing into the top corner of the net.
The Ewe Rams came out with attacking intent at the start of the second half and Finch made a superb save to tip Amy Sims' shot onto the crossbar before Barry put the rebound over.
Derby continued to dominate possession and knock on the door for the equaliser.
However, Derby were thankful that Hannah Ward cleared a Grace Foy shot off the line.
Barry's deep cross found the head of Wilson who was denied by the reflexes of Finch. From the resulting corner a Chorley defender cleared a header off the line.
Hilliard's cross then hit the top of the crossbar.
Finch saved twice from Marli Rhodes-Andrews before Sims shot wide a Wilson cross at the near post.
Gaspar was also denied by Finch at her near post.
The Ewe Rams found an equaliser in the second minute of added time when Hilliard's free kick from near the half way line beat everyone and floated into the back of the net.
There was still time for Derby to search for a winner with Rhodes-Andrews shooting wide a one-on-one opportunity and the hosts putting their bodies on the line to stop a goal.
The game though did end level and the game therefore entered extra-time.
Derby dominated both periods of extra time.
Sims shot straight at Finch after Barry teed her up inside the penalty area.
Barry then curled wide from close-range after playing a nice one-two with Rhodes-Andrews.
Molly Sutherland's shot forced a great save from the goalkeeper.
Derby got their reward their pressure deserved in the 115th minute. A corner fell to Anita Gaspar who turned and fired home into the roof of the net.
Report taken from the Derby County Women website.
