His Rolodex includes some of the choicest clients around, including UBS and Goldman Sachs, both of which are also Sullivan clients.
In the past 12 months Emmerson has advised Goldman Sachs on Macquarie bank's offer for the London Stock Exchange, and also on Goldman Sachs' £5bn ITV takeover offer with Apax Partners and the Blackstone Group.
City lawyers speculated that it was the Goldman Sachs relationship in particular that precipitated Emmerson's move. "Goldman Sachs use Sullivan all the time and they use Milbank only some of the time," said a corporate partner at one City firm.
Another partner, who knew Emmerson from his time at Freshfields, said: "This is a terrific move for Tim and a great hire for Sullivan. To be frank, it's the kind of place he should have gone to in the first place: it's a better fit for his skill set and clients."
It is understood that Sullivan has been looking for some time for a suitable lateral hire in its corporate department.
"It makes sense that they would have taken on someone with strong ties to Goldman Sachs," added the latter source.
His arrival takes Sullivan's partner headcount in M&A in London to 12, a strong platform compared with the firm's US competitors' in London: Skadden Arps Slate Meagher & Flom has nine M&A partners, while Davis Polk & Wardwell has five.
Milbank, meanwhile, is left with just one M&A partner in London in European head of M&A and private equity Michael Goroff, who splits his time between London and Frankfurt.
For now Milbank London managing partner Phil Fletcher is sanguine about Emmerson's departure.
"In the world of M&A, having client relationships is less important than being in the right place at the right time," said Fletcher. "I think we're still positioned well to get those opportunities without Tim, although we wish him well with his new venture."