Steve Hilton quit Downing Street in 2012, and later came out against his former boss over Brexit, but said that immigration had been "a source of contention" while he was working in Number 10.
In an interview with think tank Bright Blue, Hilton said that May's views were determined by the target of reducing net migration below 100,000, leaving the former home secretary with a "particularly restricted" mindset.
"That's why she was always looking for ways to restrict immigration, however beneficial the immigration might have actually been in the long-term, for example in relation to entrepreneurs," Hilton said.
"This is one of the particular arguments that she and I had; over what we wanted Number 10 to do on entrepreneurs. I had this view which was let's pursue a strategy on immigration which was based on a notion of quality not quantity."
He added: "I would say I don't agree with the strategies she's pursuing".
May has repeatedly committed to the sub-100,000 net migration target since becoming Prime Minister, and has further stressed that students should be included in the target, despite protests from her own ministers.
However, Hilton also endorsed May's desire to focus on "ordinary working families" or groups deemed to be "just about managing".
"You can say well, perhaps it's the case that the Cameron version of modernisation put more emphasis on the people right at the bottom and the Theresa May version puts more emphasis on those who are not right at the bottom but are struggling.
"I think that that's a really healthy debate and we should be pleased the Tory party is arguing about whether we help the really poor or those who are just above them and struggling."