Leslie, the candidate for Bristol North West, starts this week as an education associate.
She is a former policy adviser to shadow cabinet minister David Willetts and previously worked for Policy Exchange's education research practice.
Leslie is the second Tory hire for Tim Allan's agency in recent months, following in the footsteps of George Eustice, former press secretary to Tory leader David Cameron. Eustice joined Portland as associate director earlier this year (PRWeek, 27 March).
The Bristol North-West constituency is a key target for the Tories in the next election.
In 1992 the seat was held by the Conservatives by just 45 votes, but it became heavily Labour after 1997 through a combination of the Blair landslide and unfavourable boundary changes. The sitting MP is Labour's Doug Naysmith.