While The Beatles were on tour in North America, a Mellotron was delivered to Kenwood, John Lennon's home in Weybridge, England.
Lennon had seen a Mellotron for the first time on 9 August 1965, while producing The Silkie's version You've Got To Hide Your Love Away at London's IBC Studios.
The Mellotron had a range of 'sampled' sounds stored on magnetic tapes, which had been recorded at IBC in 1964. Lennon was intrigued and impressed with the instrument, and immediately ordered one in black.
The instrument was used on a number of recordings by The Beatles from 1966 onwards, perhaps most notably in the introduction to Strawberry Fields Forever.
Just to clarify -
Lennon's Mellotron was his home instrument and was first used by them to make some of the tape loops for Tomorrow Never Knows - Brass and string section sound swells.
His personal instrument was never used on Beatles albums after this. It did appear on Two Virgins, and the Lost Lennon Tapes though. Other Mellotrons were hired in for the music of Sgt. Pepper, Magical Mystery Tour, and the White Album.