Walter’s mother, Charlotte, was the daughter of an MP. One of her brothers also became an MP and subsequently Speaker of the House of Commons, while another became Bishop of Salisbury. Staunch Liberal supporters, the Phillimores were on close terms with Liberal leader and four-time Prime Minister William Gladstone, whom Robert had met at university. When Walter married (in Westminster Abbey) in 1870, during Glastone’s first premiereship, only unexpected government business prevented the Prime Minister from attending.

Phillimore went to Westminster School, and then to Christ Church, Oxford, which had been his father's college. He studied classics and obtained a first class degree. He then studied law and modern history (naturally, his subjects included civil law), and proved to be an outstanding jurist: he secured another first, won the University’s Vinerian Scholarship in Law, and was elected a fellow of All Soul's College. Phillimore was called to the Bar by Middle Temple in 1868. He joined the Western Circuit, whose common law practitioners argued cases in the local courts of Bristol, Exeter, Taunton, Winchester and other county towns in the West and Southwest of England. But Phillimore, like his father, made his career in civil law tribunals. With his great intelligence and family connections, he was, not surprisingly, enormously successful. There were two distinct sides to his practice. The first was ecclesiastical law, which was concerned with issues of governance, practice, ritual, and discipline within the Church of England. These were matters of intense personal as well as professional interest to Phillimore, who was a devout member of the Church, with strongly traditionalist leanings. Church law dominated the early part of his career. Ecclesiastical disputes were generally decided in the Church's own Courts, and tended only to reach the Royal Courts on appeal or review. In consequence, they made up a very small proportion of the cases which appeared in the law reports. Nonetheless, Phillimore's name featured regularly in the reports in the 1870's and 1880's in a steady flow of ecclesiastical cases.

In 1883, Phillimore did not become Queen's Counsel. The 'Law Times' thought that he had done, mistakenly listing him alongside another future Commercial Judge, Gainsford Bruce, as one of the new QCs. But Phillimore had in fact been granted a patent of precedence. This was an old and almost redundant award, invented for the benefit of barristers who wanted a QC's rank without adopting the formal status. It was believed that it had originally been invented for the benefit of barrister MPs and barristers whose practice included acting against the government. There had once been a rule that an MP who became a QC must resign from the Commons, and, in theory, a QC could not act in cases against the Crown without special dispensation. But these old notions were of no practical significance by the 1880's, and it is not entirely clear why Phillimore received the last of all patents of precedence, rather than simply becoming a QC. In any event, it made no real difference: his status as leading counsel was beyond doubt, to the extent that law reports sometimes automatically added "QC" after his name. The 1880's also witnessed the first and second of Phillimore's three unsuccessful attempts to enter Parliament as a Liberal MP, at the 1885 and 1886 general elections. He failed again in 1902, and then gave up. He did, however, become Mayor of Kensington in 1909.

The framers of the Judicature Acts had thought it appropriate to lump the old Admiralty and family jurisdictions together in a single Division of the new High Court, because both had civil law origins. However, while this made sense in theory, there was little connection in practice between the rules about annulment of marriages or adoption on the one hand and the law of maritime collision and jettison at sea on the other. In terms of subject matter, Admiralty work was much closer to the common law shipping cases which were tried in the Queen's Bench Division. It was not unusual for common law barristers with commercial expertise to appear in Admiralty cases. Some Admiralty practitioners went the other way, and Phillimore was among them. From the 1870's, he made occasional appearances in Queen's Bench shipping cases. (In Watson v GWR (1880) 6 QBD 163, for example, he led one J.C. Mathew in an unsafe dock action.) As his practice came to be increasingly dominated by Admiralty work, so his ventures into the Queen's Bench became more frequent. When the Commercial Court was created as part of the Queen's Bench in 1895, he briefly became one of its more prominent leading counsel, with a good run of appearances in the earliest volumes of The ‘Reports Of Commercial Cases’. He was a highly effective advocate, with an urbane and calm manner, and a reputation for never losing his temper, whatever the provocation from opposing counsel or the Bench.

And so, for all of his academic brilliance, Phillimore did not flourish as a Queen's Bench Judge. His occasional stints in the Commercial Court, where the law was more familiar and where he was not troubled by juries, must have been welcome relief. But they were only occasional. Phillimore did not become one of the regular Commercial Judges, and he tried only a handful of Commercial Court cases, of which Kruger v Moel Tryvan [1906] 2 KB 792, on a charterer's liability to indemnify the shipowner in respect of bill of lading liabilities, was probably the most significant. Perhaps Phillimore was considered too cerebral for the briskly pragmatic approach to which the Commercial Court aspired. With William Kennedy already slowing down business by worrying about every point, the Court did not need another Judge who might be inclined to analyse each pleaded issue until he had fully resolved it to his own intellectual satisfaction. (In fairness, however, Phillimore's general reputation was as a Judge who got through business at a commendable pace.) And possibly the perception, originally ascribed to a schoolmaster at Westminster, that all of the Phillimores were fiendishly clever, but none of them had any common sense, limited his opportunities in the early Commercial Court, where common and business sense were highly prized.

In Continental Tyre Co v Daimler [1915] 1 KB 893, Phillimore was a member of an expanded Court of Appeal which ruled that an English subsidiary of a German company was not an enemy alien, notwithstanding that its directors and shareholders were German. As a principled application of the legal concept that a company has a personality independent of its officers and shareholders, the conclusion was hard to fault. But it would probably have been difficult to explain this to the wartime British public, and the House of Lords overruled the decision ([1916] 2 AC 307). On the other hand, Phillimore's dissenting view in Beal v Horlock [1915] 3 KB 627 that further performance of a British seaman's contract of service was rendered impossible when his ship was seized in a German port on the outbreak of war was endorsed in the Lords ([1916] 1 AC 486) - with the unhappy consequence that the sailor's right to wages was terminated.

But while Phillimore did scholarly work in some difficult cases, his time in the Court of Appeal was short. He retired in 1916, aged seventy-one, without having had the opportunity to make much of an impression as an appellate Judge. Any objective assessment at this stage would surely have been that, for such an intellectually gifted lawyer, Phillimore had had a disappointing judicial career. Fortunately, the best was yet to come. In June 1918, he was made a member of the House of Lords. It was an honorary peerage, rather than a working position as a Lord of Appeal in Ordinary. But peers who had formerly been Judges were usually asked to sit on at least the occasional Lords and Privy Council appeal. This was partly a matter of courtesy, and partly because there were usually too few Lords of Appeal to get through the work unaided without building up a backlog of cases. Phillimore sat for the first time in July 1918 in London County Council v Galsworthy [1918] AC 851, a planning case. Within a year, he had participated in more than a dozen appeals in subjects ranging from collisions at sea, part of his own Admiralty speciality, to murder.

Phillimore was in his element in the Lords. There were none of the distasteful criminal trials which he had been forced to endure in the Queen's Bench. There were no juries, and no witnesses, just interesting points of pure legal principle. And the pace of work was less furious than in the Court of Appeal: the Lords heard fewer cases, and took their time in deciding them. Down to the end of 1928, Phillimore sat on well over a hundred House of Lords and Privy Council appeals.

Phillimore wrote about civil law as well as practising it. He produced new editions of his father's works on ecclesiastical and international law, and wrote a book of his own on church law. He was also active in Church affairs.

The Phillimore family was wealthy, owning substantial tracts of prime land in Kensington. (Indeed, it was suggested, perhaps only half in jest, that Phillimore had never needed to work, and that he had only joined the Bar because his father had been a Judge, and he thought that it would be convivial if he could become one too.) Phillimore eventually inherited a large part of the estate, and lived in comfortable (semi-)retirement at Cam House, a mansion with extensive grounds within Holland Park. He was married for nearly sixty years to Agnes Lushington, a niece of Dr Stephen Lushington, a great civilian lawyer of the 19th Century, who had been Admiralty Judge for nearly three decades. They had three sons and three daughters. Agnes died in early 1929. It was thought that Phillimore's grief hastened his own end, for he had remained physically fit before then, and looked twenty or thirty years younger than his real age. He died at Cam House on 13th March 1929. Henry Josceline Phillimore, who became a High Court Judge in 1959 and later a Lord Justice of Appeal, was a relative from the next generation of the family.