InHouse PR will work with the London-focused charity for the next couple of months to try and stop a drop in donations during the difficult economic period.
The mayor of London Boris Johnson has agreed to serve food at one of the charity's hostels in December to raise awareness of the issue.
InHouse PR's account covers media relations and public affairs work. The agency's recently formed InHouse London division is contacting MPs and Lords, with a survey that shows that 80 per cent of the public think the government should do more to help those at risk of house repossession.
The charity is asking the Government to do more to help the increasing number of people who are facing housing difficulties due to economic conditions.
In particular it wants the Government to ensure that people who have mental health problems are not sleeping on the streets.
The charity is also launching a direct mail and advertising campaign in January on the back of the Christmas campaign, which InHouse London will advise on.
St Mungo's appointment comes weeks after Shelter handed a twelve-month account to Bright Young Things.