"At the same time, it's undeniable that Wellington still has pockets of very visible social issues such as chronic homelessness, alcoholism, child poverty. It's a pretty grim paradox", Cassels said.
The money will come from developments in the pipeline like Erskine College, inner city apartments, and the controversial Shelly Bay site.
"People in these developments could rightfully feel that they were a part of a development that had some social responsibility about it and that's something we're starting to focus on a lot more.
"The outcome of these developments should be positive in many ways rather than just providing some houses for some people and a developer making some money", Cassels said.
The pledge comes as Wellington City Mission looks to develop a commercial building on Oxford Terrace it purchased earlier this year.
The plan is to move its services from Newtown to the new Mt Cook site.
Between 30 and 50 supported-living units will also be developed under the Housing First initiative, subject to final design plans.
The money from The Wellington Company was "huge" for the charity, Wellington City Missioner Murray Edridge said.
It would assist the Mission in its work with more than 500 of the capital's most vulnerable people.
"Through whatever causality, their circumstances are such they can't enjoy the things that most of us take for granted and in fact putting food on the table is hard, getting appropriate accommodation is hard, living what we would all describe as successful lives is hard."