NASA's Mars exploration program has turned up some evidence that the red planet may once have been at least partially habitable, though not necessarily for the kind of life forms an Earthling would expect to see on a habitable planet. For example, the Opportunity rover, which began exploring Mars in 2004, has found that pH-neutral water may have been present on parts of the planet's surface billion of years ago, which leads some scientists to believe that microbial life may have been able to survive on Mars. Another Mars rover, Curiosity, also found evidence that life-sustaining water still exists on the planet, mostly underneath Mars's rocky surface.
Curiosity and Opportunity have both found evidence that Mars could have been habitable, but as of 2015, there is insufficient evidence to confirm that there was life. Proof could come in the form of something like a fossil or other imprint of life, and while some scientists believe that they have discovered Martian fossils, these claims aren't widely believed to be true. Nonetheless, the question of whether it is possible that Mars was ever habitable seems to be answerable with a "yes."