Anthony Julius, a senior partner at London law firm Mishcon de Reya, will represent her as she negotiates a settlement with her husband of four years.
The solicitor will face Fiona Shackleton, of Payne Hicks Beach, who represented the Prince of Wales in the Royal divorce 10 years ago.
Mr Julius, dubbed Anthony Genius by colleagues, reportedly won a £17 million (€25.23m) deal for the princess in 1996. It is understood that this was his only other divorce case.
Mr Julius specialises in media law and has represented high-profile clients including newspaper proprietor Robert Maxwell.
After the death of the princess he chaired the memorial trust set up in her name, but controversy erupted over legal fees — some £500,000 — charged by his firm.
McCartney’s fortune is believed to be over £800m (€1.2 billion), and it has been reported that Ms Mills-McCartney could receive as much as £200m (€297m).
The news came as a source close to the former dismissed stories that he had deliberately locked his wife out of his London home as “rubbish”. The source said her struggle to enter the property were caused by “a simple misunderstanding”.
Police were called to the home of the ex-Beatle after a person — reported to be his estranged wife’s security guard — was found on the premises.
Ms Mills-McCartney and the couple’s daughter Beatrice were understood to have been shut out of the house after the locks were changed. When she arrived she could not get in and after her driver tried the intercom and got no answer her guard scaled the wall.
Fearing he might be an intruder, members of McCartney’s staff called the police and officers went to the address.
A Scotland Yard spokesman said: “Officers attended and spoke to a female who subsequently left the scene. No offences took place.”