In 1927, Rothwell started his own legal office in High Street, Lower Hutt, as a sole practitioner. He gradually took on partners, initially John Stanhope Reid in the 1930s, then Keith Gibson joined in 1945 to become Rothwell, Reid & Gibson. After Reid left the firm to become a diplomat, Wellington lawyer Babe Page joined the firm in his place which then became Rothwell, Gibson & Page, and now operating from both Lower Hutt and Wellington. They were joined by Jack Marshall, a Member of Parliament and later prime minister, whence the firm was then known as Rothwell, Gibson, Page & Marshall. Rothwell left in 1956 after he had been appointed the Chief Judge of the High Court of Western Samoa. His period in Samoa began when judge Charles Croft Marsack was on furlough in New Zealand. Rothwell's position was created in combination with Marsack, who was finding the work as a sole judge of a high court too onerous to be carried out alone.
