He was born as Harold Thomas Gregson in Wavertree, Liverpool, England, of Irish descent, where he was educated at the St Francis Xavier School. He met his wife, the actress Thea Gregory, when they were appearing together in rep at Perth in 1946 and they were married in London in 1947. They had three daughters and three sons.
He was credited as John Gregson in 40 films between 1948 and 1971 and on television from 1960 until his death. He was often cast as a police inspector or as a navy or army officer, or for his comedy roles in Ealing and other British films. His most famous comedy role was in the film chosen for the Royal Command Performance in 1953, Genevieve, also starring Kenneth More, Dinah Sheridan and Kay Kendall. More was described as his "rival" in British cinema at the time, although Gregson tended to appear in fewer comedy films and more dramas than More. He also appeared in the Ealing comedies Whisky Galore!, The Lavender Hill Mob, and The Titfield Thunderbolt. His best known drama films include The Battle of the River Plate, Angels One Five and Above Us the Waves. He was featured in The Treasure of Monte Cristo and had a role in Treasure Island. Gregson concentrated on TV from the mid-sixties onwards, starring as Commander George Gideon in the series Gideon's Way (known as Gideon C.I.D. in America). He also appeared in The Saint with Roger Moore, and a popular comedy adventure series with Shirley MacLaine, Shirley's World. He took over from Kenneth More in long running TV adverts for coffee on British television.
He appeared in It's The Geography That Counts, the last play at the St James's Theatre before its closure in 1957.
John Gregson died suddenly from a heart attack in Porlock Weir, Somerset, aged 55. He left a widow and six children. His final television role was in the BBC serial Dangerous Knowledge, which was broadcast posthumously in 1976.