Holman Fenwick Willan has secured a previously elusive US tie-up, with the UK top 30 firm set to merge with Houston's Legge Farrow Kimmitt McGrath & Brown.

The merger between the 450-lawyer UK firm and the roughly 20-lawyer Texan civil litigation firm will go live on 3 January next year.

The deal will be structured as a full merger, with Legge Farrow to switch from its cash-based accounting and calendar financial year to Holman Fenwick's accruals system and 31 March year-end.

The combined firm will have 170 partners and 17 offices across the UK, Europe, the US, Latin America, the Middle East and Asia. It will operate as Holman Fenwick Willan globally, including in the US.

Legge Farrow managing partner Gerard Kimmitt will remain in charge of the practice in Houston. The US firm will not initially have any positions on Holman Fenwick's 10-strong management and strategy boards, although Holman Fenwick senior partner Richard Crump said this will be reviewed in future.

Crump (pictured) said the UK firm had been seeking to establish a presence in the US for several years to satisfy growing demand from existing clients. He said Houston is an important market for the firm's energy practice – one of six core industry groups, alongside aerospace, commodities, construction, insurance and shipping.

"We looked at various US cities and wanted to find the right firm that was similarly aligned in one or more of our key sectors," Crump said. "It's great to be famous for something, but we're not just a shipping firm."

Legge Farrow name partner Glenn Legge said the merger will allow the Texas firm to "more effectively compete" for higher-value matters from existing multinational clients, including Exxon Mobil, oilfield services company Schlumberger, and Brazilian state-owned oil giant Petroleo Brasileiro.

"Right now, our sweetspot on litigation is $200m or less under dispute – anything larger than that and [clients] will gravitate towards a larger firm," Legge said. "We bring more to the table, in substance and appearance, by being part of a larger firm."

Legge said the firm has held advanced merger talks with three other international firms during the past four years – two based in the UK and one in the US – and twice got to the point of exchanging financial information and signing non-disclosure agreements, before calling off the deals due to "differences" in approach. "It would not have worked if all the elements weren't aligned, including the personalities," he said.

The structure of the combination as a conventional merger differs from recent leanings towards Swiss verein deals. Almost all recent cross-border law firm mergers have utilised the Swiss verein – a holding structure that allows members to join forces yet retain their existing forms.

This allows firms to sidestep issues around differences in profitability and tax, accounting, and partner compensation systems. Those issues can be particularly challenging in transatlantic combinations, with the cash-based accounting systems and calendar financial year favoured by most US firms clashing with the accrual-based accounting setups and 30 April financial year-end commonly used by UK firms.

Crump said the firm never seriously considered using a verein. "I'm sure the vereins are very successful but philosophically and culturally, we like to be one worldwide profit-sharing firm," he said. "We see a big advantage in terms of cohesion, collaboration, client service and culture in being one firm."

The integration of the two firms' IT, accounting and other systems is "on track" for the launch in January, Crump said. Holman Fenwick's oil and gas head, Paul Dean, who led the merger talks, will spend one week per month in Houston for the next two years, while there will be a cross-management review every three months in order to assess progress and monitor the level of cross-selling between the two legacy practices.

The two firms have maintained a close relationship since appearing on opposite sides of a dispute relating to a collision between two ships in 2007.

Legge said that Holman Fenwick clinched the deal by inviting Legge Farrow partners to its annual partner conference, which was held last month in Barcelona.

"It was a great experience and was probably one of the things that really tipped us to going with this merger without reservation," Legge said. "We were invited under the tent and there was no guarded approach – everything was exceedingly open."

Also key, Legge said, was an agreement that the merger would not upset Legge Farrow's rate structure. "We didn't want to go from Legge Farrow one day to Holman Fenwick the next day, accompanied by a marked increase in rates, which we've seen when foreign firms have decided to create a greenfield presence in Houston," he said. "That was a very big issue for us and is hugely important to our clients – particularly in the energy market, where depressed commodity prices have made the sector extremely cost conscious."

The Legge Farrow merger continues a period of considerable international expansion for Holman Fenwick. The firm has agreed tie-ups with local firms in China, Kuwait, Lebanon, Saudi Arabia and Singapore during the past 18 months.

Crump said the firm will now seek to expand across the US – probably through additional mergers. He cited New York, Los Angeles, San Francisco, Miami and Chicago as cities that have "significant relevance" to the firm's clients.

"This is a statement of intent – we don't want to stop at a presence in Houston," Crump said. "It's not some grandiose thing about wanting to be in 58,000 jurisdictions – it's about looking at where our clients are going and trying to remain relevant to them. It's a key part of the firm's strategy."

Holman Fenwick's revenue grew 3% in the last financial year to £143.1m, while profit per equity partner rose 5% to £519,000 ($645,000).