The patriarchs of the two families – 96-year-old David Rockefeller and Jacob Rothschild, 76 – cemented a five-decade acquaintance as the younger man's London-based £2bn RIT Capital investment trust bought a 37 per cent stake in the American's business.
In addition to bringing together the two doyens, the deal will considerably expand the vast networks of both families.
To give a taste: Lord Rothschild's son, Nat Rothschild, is a well-known entrepreneur with stakes in a range of companies such as Genel, the Kurdistan-focused oil producer run by former BP chief executive Tony Hayward and Bumi, the Indonesian mining group. He was also linked with George Osborne and Peter Mandelson at a notorious party on an oligarch's yacht off Corfu in 2008.
Lord Rothschild's niece Kate is married to Ben Goldsmith, brother of Conservative MP Zac Goldsmith and Jemima Khan and son of the late billionaire business tycoon Sir James Goldsmith.
On the Rockefeller side, for starters, David's granddaughter Ariana is a successful fashion designer who married the construction heir Matthew Bucklin in 2010. And let's not forget the founders who, although no longer with us, live on through their business creations.
The Rockefeller family's journey to vast riches began in 1870 when John D Rockefeller set up Standard Oil and went on establish a fortune that is widely regarded to be the largest in US history. The family's wealth management operation was set up in 1882 to manage that fortune.
The Rothschild dynasty goes back even further, to when Mayer Amschel Rothschild started a business in Frankfurt towards the end of the 18th century. It helped finance Britain's war against Napoleon in the 19th century and raised funds for a loan allowing the British government to buy the Suez canal.
The Rothschilds bought the stake in Rockefeller from French banking group Société Générale for an undisclosed sum. SocGen, which has owned the stake since 2008, appointed a new head of private banking in March, replacing Daniel Truchi with Jean-François Mazaud.
SocGen's sale of the stake comes as banks around the world sell peripheral assets to reduce their risks and strengthen their capital bases in order to meet tough regulations aimed at preventing a repeat of the 2008 financial crisis.
Commenting on the deal, Mr Rockeller said: "Lord Rothschild and I have known each other for five decades. The connection between our two families remains very strong. I am delighted to welcome Jacob and RIT as shareholders."