The music icon said that he started to "suspect everyone" close to him as intimate details of his private life started appearing in the press.
"There would be stories about how I was going on holiday to the Bahamas or whatever and I would know I hadn't told anyone," he told The Times.
"The worst thing is that then you suspect everyone - your PA, who you thought was a great girl - 'What if?' At the time of the divorce, I realised there was quite a possibility of many people hacking me for various reasons."
McCartney added that he is still wary of using the phone after becoming aware of Mills's claim that voicemails from him had been hacked by Mirror Group Newspapers.
"I used to talk on the phone and say, 'If you're taking this down, get a life'. It is a pity not to be able to talk freely on a private call," he said.
"I tend not to say much on the phone now. If I leave a message, it's benign. You edit yourself according to the new circumstances of the new world. I think it would be quite good to get some sort of laws."
Former Daily Mirror editor Piers Morgan argued ahead of his appearance at the Leveson Inquiry that it was in fact Mills who hacked McCartney's phone.