Sir Philip Mainwaring was born in 1589, the 7th son of Sir Randle Mainwaring (1553 – 1612) of Over Peover, Cheshire and Margaret Fitton ( - 1612), daughter of Sir Edward Fitton of Gawsworth, Cheshire. He was educated at Grays Inn 1609; Brasenose College Oxford 1610 - 1613; and travelled abroad. He was knighted on 13 July 1634. Sir Philip never married.
As a younger son, Mainwaring had his breeding at Court and in foreign parts, and became the first of the family to enter Parliament, though he is not known to have held office until Sir Thomas Wentworth appointed him secretary of state in Ireland at the age of 45. In the Civil War he has to be distinguished from his parliamentarian nephew, Philip Mainwaring of Baddeley, the father of Sir Thomas Mainwaring. Nothing is known of his own activity, but under the Commonwealth he was regarded as a royalist delinquent, and imprisoned in 1650 for remaining in London contrary to the Rump ordinance. He was released in October 1651 on £500 bail and took no known part in royalist intrigue.
On the eve of the Restoration, Mainwaring applied to Sir Edward Nicholas, as one secretary to another, for the master ship of the Charterhouse:
"When you have leisure to mention so poor a creature to the King, you will say that I have served him in what has been most useful, constant prayer. The result is reward enough for an old man. Yet when the sun approaches everything feels his virtues; so if I, at 72 years old, though in good health, do not put him up my pretence, I shall slip my time."
He was unsuccessful, and an attempt was even made to persuade him to surrender his Irish post on the grounds of age, but he stood firm. He was returned for Newton on the Legh interest at a by-election in June 1661.
His only committee in the Cavalier Parliament was on the bill to make better provision for curates. He died in London on 2 August the same year.
Picture shows Thomas Wentworth, 1st Earl of Strafford and Sir Philip Mainwaring by George Vertue, after Sir Anthony Van Dyck line engraving, 1739
