From wikipedia:

Several radio drama series were produced in North America and Britain. The earliest was for Radio Eireann in 1940 and starred Terence De Marney. Both NBC and CBS produced Saint series during 1945, starring Edgar Barrier and Brian Aherne. Many early shows were adaptations of published stories, although Charteris wrote several storylines for the series which were novelised as short stories and novellas.

The longest-running radio incarnation was Vincent Price, who played the character in a series between 1947 and 1951 on three networks: CBS, Mutual and NBC. Like The Whistler, the program had an opening whistle theme with footsteps. Some sources say the whistling theme for The Saint was by Leslie Charteris, while others credit RKO composer Roy Webb.

Price left in May 1951, replaced by Tom Conway, who played the role for several more months. His brother, George Sanders, played Templar on film. The next English language radio series aired on Springbok Radio in South Africa between 1953 and 1957. These were fresh adaptations of the original stories and starred Tom Meehan. Around 1965–1966 the South African version of Lux Radio Theatre produced a single dramatization of The Saint. The English service of South Africa produced another series radio adventures for six months in 1970-1971. The next English language radio series was on BBC Radio 4 in 1995, starring Paul Rhys.