The environmental charity has briefed Mischief to run two brand activation campaigns in April and July, as well as providing press office support during 2010.
The charity wants to encourage a more diverse audience, particularly young families, to visit the charity's 300 properties nationwide.
Mischief will target national and regional media, and the campaigns will have a strong digital element.
Mitchell Kaye's agency won the work following a seven-way pitch. Creative director Daniel Glover will head up the account, reporting to head of comms Andrew McLaughlin.
The activity is part of the charity's Time Well Spent marketing initiative, and comes as The National Trust unveils a brand refresh, designed to make it more 'relevant'.
McLaughlin said: 'We're trying to relax our look a bit more. We want to be more approachable and relevant in a contemporary world. Lots of people think "The National Trust is a good thing, but it's not my thing". We want to change that,' he said.
As part of this, the charity announced an overhaul of its members' magazine last month. Actor Johnny Depp will be on the cover of the first issue.
Meanwhile, the charity has reappointed Forster to work on its supporter engagement campaign Food Glorious Food. The summer campaign aims to encourage family audiences to eat more sustainably produced local and seasonal food, and to grow food at home.
This year's campaign will launch in May half-term, targeting its usual family audience. But this year, it will also target 'out and abouts', a group of mostly female adults aged 25 to 40 who attend events primarily for social reasons.
Picture credit NTPL Andrew Butter