As a foundation practice, mindfulness is areligious and that is the primary way we use it. As a clinical technique for improving our lives, and as an integral component of Qigong, Neigong, yoga, and the martial arts, mindfulness is areligious. Mindfulness is a scientific approach to self-improvement and each practitioner is his/her own laboratory. At this level of training, our goal is to be "present" when we engage in any and all forms of stillness and movement. We want to be present to every detail of our Taiji form, as well as every little detail of making a cup of tea or walking to the mailbox.
As a spiritual practice, mindfulness is again a foundation. But, like any foundation we build our structure upon it. Our curriculum of meditation and spirituality is Kriya Yoga. To that end, a Kriya Yogi typically follows the outline of Patanjali's Yoga Sutras which specifies the eight steps of Yoga. The sixth of these eight steps is Dharana, or concentration, one-pointedness of mind--otherwise known as mindfulness. So to proceed along the Raja Yoga path toward Samadhi, mindfulness is a crucial skill.
The beauty of this is that one can practice mindfulness as a life skill without practicing Qigong, Hatha Yoga, Taiji, or Yogic Meditation. On the other hand, if one is interested in any of these other paths, mindfulness is a great added benefit.
For further insight into this topic, I recommend a blog entry I did a couple of years ago, here; and/or this essay by Swami Jnaneshvara Bharati here.