Born in 1978, Umunna is the son of a Nigerian father originally from Awka. His father, Bennett Umunna, was a businessman and a director of Crystal Palace FC and was killed in a road accident in Nigeria in 1992 while running for the governorship of Anambra State. His lawyer mother, Patricia, is the daughter of the late High Court Judge Sir Helenus Milmo. Umunna was educated at Hitherfield Primary School in Streatham, South London, at the voluntary aided Christ Church Primary School in Brixton Hill, and at the independent secondary school St Dunstan's College in Catford. As a child he was a chorister at Southwark Cathedral, and participated in singing the theme tune of the British comedy series Mr. Bean. He graduated LLB in English and French Law at the University of Manchester and then studied for one term at the University of Burgundy in Dijon, before going on to study at Nottingham Law School for a Master of Arts degree.


Having completed his studies in 2002, Umunna started his career as a solicitor at Herbert Smith in the City of London, which mostly acts for large companies. In 2006, he moved to the Central London law firm Rochman Landau to specialise as an employment lawyer, where he mainly acted for individuals and small companies.

From 2006 onwards, Umunna began to write and provide commentary on the Labour Party, with a particular emphasis on economic and social issues. Umunna would usually do this in his capacity as a Member of the Management Committee of Compass, a centre-left pressure group and think tank. He wrote for the Financial Times, Tribune, The Voice, The Guardian and the New Statesman, and he began to appear on television and radio with increasing regularity, including on The Daily Politics and Question Time. He founded and is the former editor of the online political magazine, TMP, which is primarily aimed at Labour-supporting ethnic-minority Britons. Contributors to the site include Lady Prosser, and MPs David Lammy, Keith Vaz and Jon Cruddas. In 2007, Umunna worked as a campaign aide to Cruddas during his ultimately unsuccessful campaign to become the Deputy Leader of the Labour Party.

Umunna was formerly a trustee of the Anthony Bourne Foundation and the 409 Project. He is still a member of the Board of the Generation Next Foundation, a youth charity, alongside Martin Offiah, Leo Ihenacho and Rikki Beadle-Blair. In August 2007, Umunna was quoted in the wake of the numerous killings of teenagers in Britain linked to gangs and violence in urban Britain. His comments that the problems of young people living in the inner cities are linked to the wealth divide and increasing consumerism were widely reported. He challenged former Sun editor Kelvin MacKenzie for making remarks about then-Prime Minister Gordon Brown on an episode of Question Time in October 2007.

In November 2007, Umunna was identified as one of ten young, gifted and black people in British politics in The Independent on Sunday by Simon Woolley, the Director of Operation Black Vote. Woolley said Umunna "may end up as the UK's Barack Obama". In a February 2008 edition of the New Statesman, he was referred to in an article entitled "Is there a British Obama?". He was also described in the New Statesman as "a Barack Obama for Britain".The comparison with Obama was one that Umunna later came to disparage. He was also one of the individuals selected for The Observer's January 2009 Hotlist, which highlighted people set to make a mark over the coming year. In May 2009, he was chosen to be on a panel of ten figures from across the political spectrum addressing the question How do we restore the reputation of Parliament? by The Independent following the 2009 expenses scandal.