McCartney wrote the song in May 1964 while cruising the Caribbean aboard a yacht called Happy Days with his then-girlfriend Jane Asher. Music critic Ian MacDonald said, "The sombre lyric—provoked by the frustrating interruptions of a relationship between two career people—matches the lowering gloom of the music."
The song is one of three main compositions (along with "And I Love Her" and "Can't Buy Me Love") that Paul McCartney wrote for the film. Recorded in advance of film production for lip-synching reasons, "Things We Said Today" has a reverse nostalgia premise. McCartney said, "I wrote this on acoustic. It was a slightly nostalgic thing already, a future nostalgia. We’ll remember the things we said today, sometime in the future, so the song projects itself into the future."
McCartney was particularly satisfied with his chord change, F major to B flat major—instead of the more obvious F minor—which first occurs beneath "…wishing you weren't so far away" in the song.John Lennon accentuates the strident acoustic guitar strumming by triple hitting a low A note on a piano. The dynamic of the song moves from melancholy to rock and back with a minor to majorkey change during its middle eight section.
The Beatles recorded "Things We Said Today" in three takes on 2 June 1964. Take one was a false start, take two was the rhythm track, and take three was used for overdubs of the main vocal, tambourine, and piano. According to author and Beatle historian Mark Lewisohn, the piano was supposed to be omitted from the mix but is audible owing to leakage onto other microphones during the recording of the overdubs, but MacDonald was sceptical about that conclusion.
The group also recorded "Things We Said Today" twice for BBC radio, on 14 and 17 July 1964. The first, a performance for the Top Gear radio show, was featured on the Live at the BBC album in 1994. These performances took place around the same time as the theatrical release of the Beatles' film A Hard Day's Night, and on the 14 July performance the song was erroneously introduced as "one of their film songs." The song was not in the film, but was included on Side 2, the "non-soundtrack" side of the British A Hard Day's Night LP. 
The Beatles included "Things We Said Today" as part of their live set during their 1964 tour of the United States and Canada. George Harrison sang harmony vocals alongside Paul McCartney during the performances.