I got in touch with mindfulness by reading one of Jon Kabat-Zinn's books ("Wherever you go, there you are"). That was almost twenty years ago. Shortly after that I read Ellen Langer's book "Mindfulness", which is more about the negative effects of mindlessness.
I think that the pure process of observing something (mindfully) has the potential to positively influence the results. Whether I sit down quietly and observe my feelings and thoughts (things that happened in the past or things that'll happen in the future) as they come and go. Or as I interact with people, or as I walk, travel or engage in any type of activity.
I don't need scientific evidence that all of this has a measurable positive effect. My own observation is enough evidence for me.
Edit: typo