Grisham said that she saw Trump handle sensitive records.
She told USA Today that seeing him put records in his jacket pocket risked staffers not being allowed to see them.
Trump either didn't know or didn't care that this was a potential national-security threat.
Officials from previous White House administrations said that Trump's handling of documents appeared to be a troubling breakdown in the protocols meant to keep them safe.
The Mar-a-Lago residence of the former president was searched by the FBI in August. Many of the documents were classified.
According to Justice Department court documents, some of the records were so sensitive that agents had to get special clearance to view them.
Depending on the classification of the documents, how they are handled is up to you. They are only meant to be seen in secure facilities.
Highly classified records were kept with other records, newspaper clippings and scraps of Trump's handwritten notes, according to court documents.
Grisham has said that during Trump's time in office, records would "just disappear" and that he carelessly stored classified documents in boxes.
She and other former aides have talked about the treatment of official documents by Trump and his staff.
Grisham joined the Trump campaign in 2016 and went on to serve in various roles, including Trump's press secretary and chief of staff
She became a critic of the former president after quitting the administration as the Capitol riot was happening.
Trump attacked Grisham and said that she was lying about him.