After being demobilised in 1945, he joined the Playhouse in Liverpool for a year, before going on to Perth Theatre in Perth, Scotland. Here he met his Future wife, Actress Ida Reddish from Nottingham, who at the time was using the stage name Thea Kronberg (later Thea Gregory) and had recently arrived from the Birmingham Repertory Theatre. In 1947 they moved to London and married there. They eventually had three daughters and three sons.

One of his first appearances was in the film Saraband for Dead Lovers, a tearjerking romance starring Joan Greenwood and Stewart Granger. In the popular Scott of the Antarctic (1948) he played Tom Crean.

Gregson could also be seen in Ealing's Whisky Galore! (1949) (the first of the Ealing comedies) and Train of Events (1949), as well as The Hasty Heart (1949), Cairo Road (1950), Treasure Island (1950) and The Lavender Hill Mob (1951). Gregson had a good part in Angels One Five (1951), a popular war film.

After that 'his film career faded after ten good years [1952–1962]'. He was one of many leading men and women of the 1950s (the others including Kenneth More, Richard Todd, Patrick Holt, Michael Craig, Sylvia Syms and Muriel Pavlow) who struggled to maintain their status as leads beyond the early 1960s. From 1963 onwards, Gregson never played another leading film role.

Gregson became a star when cast in the comedy Genevieve (1953), also starring Kenneth More, Dinah Sheridan and Kay Kendall. It was the second most popular film of the year in Britain.

He was second billed to Glynis Johns in a prison drama, The Weak and the Wicked (1954), another hit and played the lead in a light drama, Conflict of Wings (1954). Gregson followed this with The Crowded Day (1954), a comedy; To Dorothy a Son (1954) a comedy co-starring Shelley Winters; and Three Cases of Murder (1955), an omnibus film co-starring Orson Welles.

More successful was another war movie based on a true story, The Battle of the River Plate (1956) where Gregson played F. S. Bell. This film helped British exhibitors vote him the 8th biggest British film star in the country for 1956.

He appeared in It's the Geography That Counts, the last play at the St James's Theatre before its closure in 1957.

He supported in SOS Pacific (1960) and Hand in Hand (1960) but was top billed in Faces in the Dark (1960) and The Frightened City (1961). He has a support role in The Treasure of Monte Cristo (1961) and was one of many names in The Longest Day (1962).

Gregson's final film roles of note were in Live Now, Pay Later (1962) and Tomorrow at Ten (1962).

TV work became increasingly important to him from the mid-60s. He starred as Commander George Gideon in the 26 episodes of the series Gideon's Way (1965–66) (known as Gideon C.I.D. in America).

His final television role was in the Southern Television serial Dangerous Knowledge, which was broadcast posthumously in 1976. His body was interred at Sunbury Cemetery, Sunbury-on-Thames, Surrey near his family home at Creek House, Chertsey Road, Shepperton.

Gregson was born of Irish descent, and grew up in Wavertree, Liverpool, where he was educated at Greenbank Road Primary School and later at St. Francis Xavier's College. He left school at 16, working first for a telephone company, then for Liverpool Corporation, as the city council was then known, before the Second World War. During this time, he became interested in amateur dramatics, joining first his local Catholic church theatre group at St. Anthony’s, Mossley Hill, and later the Liverpool Playgoers' Club.