The merger, which was completed on Monday, will see Depression Alliance subsumed into Mind with the loss of the alliance's identity.
A spokeswoman for Mind said the merger was a way for Depression Alliance, which became a wholly-owned subsidiary of Mind in April, to expand its services to more people.
There will be no job losses as a result of the merger, with the charity’s five members of staff at its London offices moving to Mind’s head office in Stratford, east London, the spokeswoman said.
The other 12 members of staff at Depression Alliance, which has been in operation for almost 40 years, will join one of Mind’s regional branches, and Depression Alliance’s London offices have been vacated.
The former chair of Depression Alliance will join Mind’s council of management, and the charity’s chief executive left at the end of 2015, before a merger was suggested.
A statement on Depression Alliance’s website said it would no longer be updated following the merger and instead provides links to Mind’s website. But the alliance’s Friends in Need online community will retain its own brand identity, the charities said.
According to the Charity Commission’s website, Mind had an income of more than £37m and expenditure of more than £36m in the year to 31 March 2015, and also employed 692 staff.
Depression Alliance had an income of almost £698,000 and expenditure of approximately £778,000 in the year to 31 March 2015.
Paul Farmer, chief executive of Mind, said: "I’m excited about Depression Alliance and Mind coming together. It creates a unique opportunity to increase the availability of high-quality peer support for everyone with depression by 2021. I’m looking forward to working with our new colleagues to make this happen."