Hubbard was born in Reading in Berkshire, but was brought up in Guernsey in the Channel Islands. He was educated at Elizabeth College, Guernsey and at Jesus College, Oxford, where in 1933 he won the Newdigate Prize for poetry with "Ovid among the Goths". He served with the Indian Civil Service from 1934 until its disbandment in 1947 upon Indian independence, after which he worked for the British Council and as Deputy Director of the National Union of Manufacturers. From 1960 until shortly before his death he worked as a freelance writer. He lived in Dorset and in Scotland, and was married with three children, although separated at his death.

P. M. Hubbard's main output was sixteen full-length novels for adults. These are typically suspense stories which have their settings in the countryside or coastline of England or Scotland (although one, The Custom of the Country, is set mainly in Pakistan). Most of the novels feature a male protagonist (although in some, such as Flush as May and The Quiet River, the protagonist is a woman) and characters who in general are middle-class, articulate and strong-willed. Most of the novels draw extensively on one or more of the author's interests and preoccupations including country pursuits, small-boat sailing, folk religion and the works of William Shakespeare.

Hubbard's novel High Tide was adapted for television and broadcast in 1980 as part of the UK ITV network's "Armchair Thriller" series.

He was described in his obituary in The Times as a "most imaginative and distinguished practitioner", writing with an "assurance and individuality of style and tone." He died on 17 March 1980.

Adult suspense novels

• Flush as May (1963)
• Picture of Millie (1964)
• A Hive of Glass (1966)
• The Holm Oaks (1966)
• The Tower (1968)
• The Custom of the Country (as The Country of Again in US) (1969)
• Cold Waters (1969)
• High Tide (1971)
• The Dancing Man (1971)
• A Whisper in the Glen (1972)
• A Rooted Sorrow (1973)
• A Thirsty Evil (1974)
• The Graveyard(1975)
• The Causeway (1976)
• The Quiet River (1978)
• Kill Claudio (1979)

Novels written for children

• Anna Highbury (1963)
• Rat Trap Island (1964))