The Buddhist technical term sati in Pali (or smrti in Sanskrit) was first translated with the English word mindfulness by T.W. Rhys Davids in 1881. So says Rupert Gethin in his article On some definitions of mindfulness (Contemporary Buddhism Vol. 12, No. 1, May 2011).
Previous dictionaries had translated this complex technical term with translations such as ‘remembrance, memory, reminiscence, recollection, thinking of or upon (any person or thing), calling to mind.’ (p.263)
One of the misunderstandings of the word ‘mindfulness’ is that people sometimes assume it is a Buddhist word, rather than an English word that translates a Buddhist term.
Gethin goes on to say that he is not surprised that this word is used. The OED ‘records the use of the English ‘mindfulness’ in the sense of ‘the state or quality of being mindful; attention; memory (obs.); intention, purpose (obs.).’ from 1530 A.D.’ (p.264)
Early uncertainty about how to translate the word sati gives way to mindfulness becoming ‘the only possible English translation of sati’ from 1910 with Rhys Davids influential translation of the Mahasatipatthana Sutta. (p.265) It is not surprising then, with this OED reference to the date 1530, that the King James Version of the Bible published in 1611 has the word ‘mindful’ in it a number of times.
‘What is man, that thou art mindful of him? and the son of man, that thou visitest him?’ (Psalm 8:4) Being mindful of God is becoming aware of when He ‘visitest’ us.