Sir Robert Sawyer, the attorney-general of England and Wales, was born in 1633, the younger son of Sir Edmund Sawyer of Heywood Lodge in White Waltham, the Auditor of the Exchequer, by his second wife, Anne, daughter of Sir William Whitmore of Apley, near Bridgnorth, in Shropshire.

Robert Sawyer was admitted, on 20th June 1648, as a pensioner at Magdalene College, Cambridge, where he was a 'chamber fellow' of Samuel Pepys. On 16th May following, he was elected the first Craven Scholar. In 1652, he graduated a BA and was elected Goche Fellow. In 1654, he became Dennis Fellow. In the following year, Robert graduated a MA and was also incorporated at Oxford. He is numbered among the benefactors to the library of Magdalene College. After leaving the University, Sawyer was called to the bar from the Inner Temple. He was treasurer of the Inn from 1683 to 1688, and practiced in the Exchequer Court and on the Oxford circuit. On 27th November 1666, Pepys went to the House of Commons and heard Sawyer acting as counsel for the impeachment of John, Lord Mordaunt, younger son of the 1st Earl of Peterborough, and was "glad to see him in so good play". Sawyer's progress at the bar was assisted by his relationship to Francis North, Baron Guilford.

As early as 1661, Wood mentions Sawyer as an aspirant for parliamentary honours, but he does not seem to have been elected to the House of Commons till November 1673, when he was returned for Chipping Wycombe. He became a frequent speaker, more especially on legal topics, was knighted on 17th October 1677 and, on 11th April 1678, was elected speaker on the proposition of secretaries Coventry and Williamson. However, on 6th May, he resigned the office because of ill-health. Sawyer was sufficiently recovered to take part in a debate on 4th November of the same year, when he declared himself in favour of an address to the effect "that the king be humbly desired to prevail with his brother to declare in open parliament whether he be a papist or no". He assisted in drafting the Exclusion Bill, a fact which, when acting as attorney-general to James II, he naturally did his best to conceal.

On 18th July 1679, Sawyer appeared at the Old Bailey as the prosecutor of Sir George Wakeman and some Benedictine monks alleged to have been concerned in 'the popish plot' but failed to get a verdict. On 14th February 1681, he was sworn as attorney-general in the room of Sir Creswell Levinz. In June 1681, with the help of Finch, the solicitor-general, and Jeffreys, he conducted the prosecution of Edward FitzHarris; and, on 17th August of the same year, obtained the conviction of Stephen College, the protestant joiner, though the crown witnesses were thoroughly discredited. On 24th November, Sawyer prosecuted Shaftesbury before a London grand jury for treasonable association, but a bill of ignoramus was returned, when Sawyer moved that the "hollowing and hooping" which followed the verdict might be recorded. Sawyer represented the Crown on 27th April 1682, the second occasion on which the case against the City of London charter was argued. He contended that the quo warranto "was not brought to destroy but to reform and amend the government of the city". On obtaining his verdict he moved, "contrary to what is usual in such cases, that the judgment might not be recorded". Sawyer's argument was regarded by lawyers as a masterpiece. The arguments of Sawyer, with those of Finch, Pollexfen and Treby, were published in 1690.

In 1683 and 1684, Sawyer conducted the chief prosecutions arising out of the Rye House Plot, when his harshness towards Lord Russell was contrasted with the mildness of Pemberton, the presiding judge. In reference to Sawyer's contention that a copy of the jury-panel was granted to Russell not of right but of privilege, Hawles remarks that "of all men who ever came to the bar he [Sawyer] hath laid down the most rules which depend totally upon the authority of his own saying". On 7th November 1683, Sawyer appeared against Algernon Sidney; on 6th February 1684, he prosecuted John Hampden the Younger for misdemeanour; and, on the following day, obtained verdicts against Laurence Braddon and Hugh Speke on the charge of suborning witnesses to prove that Essex was murdered. On 14th June, he moved the Court of King's Bench, presided over by Jeffreys, for execution against Sir Thomas Armstrong, who had been outlawed, and obtained his immediate conviction, to his own subsequent undoing. In 1684, Sawyer acted as one of the counsel for the East India Company in their action against Sandys, in what was known as 'The Great Case of Monopolies’. He appeared against Titus Oates on 8th and 9th May 1685, and obtained his conviction for perjury. In the following year, on 14th January, he failed to get a verdict against Henry Booth, 2nd Lord Delamere, who was prosecuted in connection with Monmouth's Rebellion.

Sawyer's "bias was to loyalty, which had been the character of his family", but he was also fairly attached to the church, and he was not prepared to go to all lengths with James II in civil matters. When the question of the dispensing power arose, he told James that "in point of law the power was not in the king", and gave written reasons for refusing to pass Sir Edward Hales' patent of dispensation. Finally, however, he deferred to the opinion of the judges and signed the patent as a ‘ministerial officer’. When the patent for the confirmation of Obadiah Walker, a Roman catholic, as master of University College, Oxford, was subsequently brought to him, he objected to it "as being against all the laws since the days of Elizabeth" and "begged on his knees for his dismissal". Subsequently, he refused to pass a patent to the Duke of Berwick as lieutenant and custos of the forest. In spite of Sawyer's resistance, James retained him in office till December 1687, employing him as attorney-general when government wished to enforce the law, and Sir Thomas Powis, who had replaced Finch as solicitor-general, when the law was to be broken. Early in 1688, Sawyer acted as counsel to the Queen-Dowager in her suit against Henry Hyde, 2nd Earl of Clarendon. In June 1688 Sawyer appeared as senior counsel for the seven bishops and, in Macaulay's opinion, did his duty "ably, honestly and zealously".

Sawyer was elected to the Convention parliament for Cambridge University on 17th January 1689 and took an active part in its early proceedings. He contended that James II, by leaving the country, had ipso facto abdicated, but that the "vacancy of the throne makes no dissolution of government neither in our law nor any other"; and moved that the house should vote it "inconsistent with a protestant government to have a popish prince". Sawyer, however, being of opinion that the Convention could not grant money, moved, on 19th February 1689, "that the King be advised to issue out new writs to call a parliament". On 17th June, during the debate on the heads of a bill of indemnity, he gave a full explanation of his attitude towards James II, and declared he had "never had a pardon, nor ever desired it". But in January 1690, Sawyer was attacked by Hawles and others for his conduct in the case of Sir Thomas Armstrong. On the 20th, Mrs. Matthews, Armstrong's daughter, came to the bar and testified to Sawyer's part in the prosecution, but admitted that he had denied at the time his power as attorney-general of granting a writ of error to stay the proceedings, and she was, moreover, unable to say that he had demanded execution before the judges had declared themselves. Upon her withdrawal, Sawyer contended that he had only done his duty in putting Armstrong on trial. He then retired from the house. In the debate which followed, the lawyers seem to have been divided in their opinions, but violent speeches were made against Sawyer by John Hampden the Younger and others; and a motion was finally carried by 131 to 71 to expel him from the house. Hallam applauds the decision, but Macaulay thinks that "calm and impartial judges" would have decided in Sawyer's favour.

A month later Sawyer was again returned for Cambridge University, Sir Isaac Newton being among his supporters. He took part in the debates on the Recognition Bill and on the Regency Bill in April and May, after which his name disappears from the records. In June 1691 he "put in to succeed" Pollexfen as Lord Chief Justice and, in March of the next year, was thought likely to become Lord Chief Baron; but he died on 30th July 1692 in his house at Highclere in Hampshire. He was buried in the church which he had built there in the preceding year. By his wife, Mary, daughter of Ralph Suckles of Canonbury in Middlesex, he had one daughter, Margaret. She married Thomas Herbert, 8th Earl of Pembroke, and died in 1700. Her second son inherited the estate in accordance with his grandfather's will. After his death in 1769, Highclere reverted to the elder branch and finally became the property of the Earls of Carnarvon.

Roger North, who often assisted Sawyer when attorney-general, describes him as "a proper, comely gentleman, inclining to the red; a good general scholar, and perhaps too much of that, in show at least, which made some account him inclined to the pedantic". Though "proud, affected and poor spirited," he thought him on the whole an efficient law officer. In capital cases, Sawyer, according to North, "was very careful, and used to consult at his chambers with the King's Counsel," and in case they thought the evidence inadequate, "he never pushed any trial against any man". The whig, Burnet, characterises Sawyer as "a dull, hot man, and forward to serve all the designs of the Court". Sir John Hawles' legal criticisms, although entitled to consideration, are those of a political opponent.