Mr Thomas, who has been MP for Harrow West since 1997, told the capital’s Evening Standard newspaper that London was practically a “city state” and that it needed more powers to govern itself.
“I’m keen to push more devolution of power to London and, crucially, to Londoners. We’re at a point now where the UK is no longer just a union of England, Scotland, Northern Ireland and Wales, but actually a union with the city state of London,” he told the newspaper.
Mr Thomas held a string of ministerial jobs under the last Labour government at the department for international development and the business department.
He has served in the shadow cabinet throughout Labour’s time in opposition, most recently shadowing a foreign affairs brief.
Mr Thomas joins a growing field that currently includes Tottenham MP David Lammy, Tooting MP Sadiq Khan, Hackney MP Diane Abbott, former Olympics minister Tessa Jowell, and transport expert and Labour activist Christian Wolmar.
Who will be the next Labour leader?
Shadow business secretary Chuka Umunna has ruled himself out of running after dropping out of the Labour leadership contest.
Incumbent Mayor of London Boris Johnson, who was returned as an MP in last week’s election, is not expected to stand again.
Labour will pick its Mayor in September. Anyone who registers as a supporter will be able to vote in the primary election to choose the candidate whether or not they are full party members.
The contest was originally set for July but was delayed, with the candidate set to be announced around the same time as the new Labour leader. The full mayoral election is on 5 May 2016.