have on mind

have (someone or something) on (one's) mind

To be thinking about someone or something, perhaps to the point of preoccupation. I've had Jennie on my mind lately—do you know how she is? You look worried—do you have something on your mind?
See also: have, mind, on

have someone or something on one's mind

 and have someone or something on the brain (More informal with brain.)
to think often about someone or something; to be obsessed with someone or something. Bill has chocolate on his mind. John has Mary on his mind every minute. Karen has Ken on the brain.
See also: have, mind, on