During World War II, Longdon was a pilot with the Fleet Air Arm, protecting Atlantic convoys.

While stationed at a naval base near Blackpool, he acted in a show and was seen by actor Douglas Hurn who encouraged him to pursue it.

[2] After the war, Longdon trained at RADA (1946–48), and made his first stage appearance at the Lyceum, Sheffield in 1948, and his West End debut the same year.

[3] He was also known for his character actor roles in British television productions such as The Sandbaggers, Danger Man and The Avengers.

[5] He had a major supporting role in the 1958 film Another Time, Another Place starring alongside Sean Connery and Lana Turner.