The Sentinels were first seen in Los Angeles, where they uncovered and destroyed a mutant nest. They were built by Bolivar Trask, as the solution to anti-human attacks, and hoped that the Sentinels would decimate the mutants by 50%. Professor X and the X-Men managed to convince the President to halt the Sentinels for a time. But after the Savage Land was located, a final mission was planned for the Sentinels. They were deployed there to wipe out the Brotherhood of Mutants. However, Magneto magnetically rearranged their circuit boards to make them hunt people without the mutant gene and deployed them against Washington D.C. where they were beaten by the X-Men. Charles Xavier then turned Magneto into a super magnet and exploded him in the sky. The Sentinels appeared to have been destroyed with him.
Note: There was also a group of S.H.I.E.L.D. agents who wore Sentinel-like armour and called themselves the New Sentinels.