The firm added TUI Travel, Group 4 Securicor (G4S) and Resolution to its roster of FTSE100 clients, according to the latest quarterly figures from Hemscott.
Herbert Smith head of corporate Michael Walter said: "We've been concentrating hard on our FTSE client business and this is the result. I'm not surprised at all. It no more than reflects the rank we think we have - right up there with the best."
But the firm's achievement may prove short-lived, as one of its largest clients, Resolution, is set to drop out of the FTSE100 when a private takeover is completed later this year.
Herbert Smith has acted for the life assurer in the past, but became one of its main advisers for the first time last year.
The firm's ranking was also boosted by longstanding client G4S, which was added to the index last December.
The firm shared second place with Linklaters, with both acting for 19 of the UK's largest companies last year.
Freshfields dropped to fourth after losing the Daily Mail, ICI and Northern Rock, but gaining G4S and the London Stock Exchange.
A&O fell two places to fifth after losing BAE Systems, ICI, Mitchell & Butlers and Northern Rock. It won Kelda Group and TUI Travel.
There was no change at the top, with Slaughter and May holding on to first place despite shedding BAE Systems and Barrett Developments from a total of 28.
There were two new-comers to the Hemscott table: Scottish firm Shepherd and Wedderburn, which secured three top-100 clients to share fourteenth place, and Nabarro, which was eighteenth with two FTSE100 clients.