The long-running CBS sitcom will draw to a close at the end of its ninth season, with the identity of Ted's (Josh Radnor) elusive wife to finally be revealed.
"I think we would be forbidden from show business if we ended the series without meeting the mother, and rightfully so," Thomas joked in an interview with TV Guide.
"The audience [won't be] jerked around or shortchanged in any way," he added.
Despite the last-minute renewal - with many expecting How I Met Your Mother's eighth season to be its last - Thomas insisted that the final season will be "great" and will tell the story of Ted's quest to get married in "a new way".
"You're never going to mistake a season nine episode from one of the previous eight," he claimed. "There are a lot of loose ends and references and setups that we have to pay off.
"This is the final lap, so there's no more withholding of fun ideas. There's something very carpe diem about it. We're going for it and looking to not leave anything on the field."
Season eight of How I Met Your Mother continues on Mondays at 8/7c on CBS, with new episodes currently airing on E4 in the UK.
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Watch a clip from How I Met Your Mother below: