The first atomic clock was an ammonia absorption line device at 23870.1 MHz built in 1949 at the U.S.
The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen and Jack Parry in 1955 at the National Physical Laboratory in the UK.
A brief history of timekeeping
Innovation in Laboratory Time and Frequency, Neuchâtel
Norman Ramsey, in the origin of atomic clocks (French)