During the campaign Clegg's powerful performance in TV debates thrust his third party temporarily into second place in polls, although they slumped in the election itself, winning only 57 seats -- five fewer than in 2005.
But on Tuesday 43-year-old Clegg finalised a deal with Cameron to take office -- the first time the Liberals, as they used to be called, have been in government since David Lloyd George left power in 1922.
Clegg was elected Lib Dem leader in December 2007, just two years after he first entered parliament as lawmaker for Sheffield Hallam in northern England.
He made a name for himself early on with his strong defence of civil liberties and by breaking parliamentary taboo by openly criticising the war in Afghanistan.
His fervent support of the European Union and the euro and his international background has singled him out among British politicians, but his privileged past has also drawn comparisons with the Eton-educated Cameron.
Nicholas William Peter Clegg was born on January 7, 1967, and brought up in the affluent village of Chalfont St. Giles in Buckinghamshire, northwest of London, with two brothers and a sister.
His family history is exotic -- his mother is Dutch, having been born in Indonesia and held in a Japanese internment camp before she came to Britain aged 12, while his wealthy banker father is half-Russian.
His wife Miriam is a Spanish commercial lawyer. They married in 2000 and their children -- Antonio, Alberto and Miguel -- are all bilingual. Clegg himself speaks Dutch, French, German and Spanish.
Clegg attended London's elite Westminster school alongside a young Helena Bonham Carter, the future Hollywood star.
He got into trouble as a 16-year-old there when he got drunk and set fire to a collection of rare cacti on a school trip to Munich.
He went on to study social anthropology at Cambridge University, where he starred in a play directed by future Oscar-winner Sam Mendes, who remains a friend.
He completed his education at the University of Minnesota and the College of Europe in Bruges, where he met his future wife, the "love of his life".
Before that, Clegg enjoyed the single life -- he told GQ magazine in a now notorious 2008 interview that he had slept with "no more than 30" women, earning him the nickname "Nick Clegg-over".
Clegg worked briefly as a journalist and a political consultant before joining the European Commission where he worked for five years, including as senior aide to Commission vice president Leon Brittan, a Conservative.
He subsequently stood as a Liberal Democrat member of the European Parliament in 1999, a job he held until 2004.
At that point, finding the travelling too much for his young family, Clegg stood down and returned to England where he worked as a lecturer and a part-time lobbyist before joining parliament in 2005.